I collect several different things, and I think there are some general mistakes and some generally good practices to follow while collecting. This is all based on my personal experience of years of collecting various objects. I will try to stick to the facts and not delve into the psychological qualities of collection, because I have already written a writeup on the topic: The Joy of Collecting.
Without further ado, I shall start optimistically -- the good practices!!
This is crucial. I recommend this first and foremost, to absolutely everyone.
Using a spreadsheet is the most logical, efficient, and powerful way to do this -- however, if you don't like spreadsheets because they're too confusing, you should find a system that works best for you. The power of the spreadsheet is that once you enter your data, you can create graphs, charts, have cells that do math for you and count things, etcetera.
This is how I do it for Pokemon cards: I track card, number, full art, first edition, holographic, condition, graded, set, rarity, date acquired, and what they cost me. I can create a filter and click a button to sort by cost, or sort by set, or sort by holographic, etc. If I were in the habit of constantly buying cards, I could create a bunch of charts, such as a chart that lists how much I am spending per month (I almost never buy cards anymore though, and my collection is super small), or a chart that shows how many cards of each set I have, or holo versus reverse-holo versus nonfoil. My collection is small these days so there is no chart, but I do have a "totals" field that totals the money I've spent.
That is absolutely crucial. You need a totals field for each thing you are collecting, so you know how much you have spent.
The same goes for my watches. I track the model, the brand/manufacturer, the date acquired, and what they cost me. Or for my copies of Ovid's Metamorphoses -- I track the translator, the ISBN, date acquired, and what it cost me.
I think you should only collect a thing if you have a space for it. If it piles up on a table or gets shoved in a drawer, there is no point. You must, must curate a space wherein the collection can be reside and be displayed. I think that for a collection to occupy space in your living area, for it to be displayed and seen, allows it to hold real-estate in your psyche. My watches are displayed on my dresser above my ties. My mugs are on top of my speaker system, my copies of Metamorphoses are on the bookshelf directly ahead when you walk in.
When things are afforded a proper and appropriate space in your environment, you are curating an environment that is an expression of yourself. This is the nature of collecting. This is also the case with other ways you curate your space, such as the practice of crafting an altar or "prayer corner" in your home. Additionally, I think the desire to display a collection extends beyond mere expression of its importance or of the self. Collection is self expression, yes, but it's also a form of passive enjoyment. To be in the presence of a place of collection, to look upon the collection with pride, brings a HUGE amount of joy. A properly odered home creates a properly ordered mind.
This is important. This is perhaps less true for mugs, but for pokemon cards, and for watches, and for books -- there are often numerous sellers. You cannot determine the fair price based on a single listing. No matter what. Unless there is only a singular listing in existence, it's just not possible. For example, if I search "Flareon 10/110 NM" on Ebay, there are a HUGE variety of prices for this exact card in this exact condition. At the time of writing this, there is one for $32, $36, $77, $60, $40, $50, $86, $36. (This is ignoring the reverse holo variants and only including the cards that are the same regular holo cards from the legendary collection set.) Collectible things like watches are the same way, though you will have less luck with more niche watches. You MUST shop around.
An EXTREMELY valuable tool is to go to Ebay, go to "advanced search", and tick the little box that says "sold listings". This is especially useful if there are a TON of different price ranges, because this will tell you what the item tends to actually sell for, and not what people are trying to sell it for. It will show you the sale price of the item at the time it was purchased; sometimes it might actually be a lot lower than some of the listings. Oftentimes people will list things expensively with the expectation that people will submit offers. It's a legitimate tactic.
These are ALL traps that I have fallen into, some of them into DOZENS of times. It's a brutal cycle and being aware that these practices are bad can save you a lot of money.
This is so so so so important. You can hook your credit card up to ebay and make a purchase in a few seconds. The pictures are what draw me in. I will be scrolling and see a really gorgeous shiny awesome new thing, and I am filled with lust. In less than four seconds I can purchase it. Just push a button and BAM, $50 down the drain. And then I feel sick because I spent more than I should have becuase it looked so cool!!!!!
I have to make a conscious, deliberate effort to control my impulses. I find that it helps to do the following specifically, all on principle:
The later into the night it is, the less impulse control I have. I deliberately do not open any websites where one can make purchases after a certain time at night.
Bidding wars are real, fought and paid in tears. I raise my bid, the other guy raises his, I raise mine, he raises his, and suddenly it's a competition. My initial motive of trying to get a good price is gone; it's no longer about trying to get a deal; it's about winning. Victory.
It's a real psychological phenomenon. It's the reason people auction things off instead of selling them at their regular rate; auctions get crazy high because highly competitive people (like myself) get psychologically manipulated by each other. It's a monetarily-expensive echo chamber. It's brutal.
This is what I do nowadays on Ebay, and this is a course of behavior that I have found is conducive to the health of my wallet. I have noticed that this is problematic for me, so now I have a system. I think about the maximum price I am willing to pay, I enter that as my maximum bid, and that's it. I don't look at the auction again. I don't follow it. I don't open it. No matter how tempting it may be. Ebay will automatically bid the smallest possible bid, and respond to other people's bids, up to your own maximum bid.
ALSO -- do not forget about auctions you have already bid on!!!! It's so easy to bid on a dozen different auctions and think "I'm not going to win any of these because of how low I'm bidding" and suddenly you have five auctions to shell out for. PAY ATTENTION!!!
I don't care if you will get brownie points for "completing" a collection. If you don't want it, and it will not bring you any joy or happiness, do not buy it. Period. For example, I have four mugs that are official merch from the videogame Blasphemous. It's my favorite piece of fiction, I may end up getting a tattoo of something from the game. There are two more mugs for sale, and I COULD buy them to have a "complete" Blasphemous mug collection. But I don't care for them and would never use them to drink out of.
I can give another, perhaps better example. Pokemon cards. When I was super into it, I wanted to collect all the VMAX Eeveelutions. Just for fun, because some of them were SUPER cool. My problem, though, was that some of them I just didn't care for. I was shelling out huge amounts of money on these cards, we're talking $30 a card, and I didn't like them at all, just on principle because it would bring my collection closer to being "complete". Eventually I realized "this is stupid, why am I doing this?" And it was stupid.
There is nothing more frustrating than spending all kinds of money and then the collection just sits in a drawer or in the closet. You forget about the thing, you don't get to enjoy it actively or passively, ultimately the passion and love you have for the thing is squandered. You MUST have a space for everything you collect.
This is huge for watches because dresser drawers are kind of the "typical" place for a watch to be situated -- I had my brother 3d print a little stand for me that my watches can be displayed on. This was also me when I had a huge Pokemon collection. Now I just have four cards, cards I know I love, and they are displayed on a shelf, and they look nice. I don't plan on getting a ton of new cards, or getting rid of a bunch. This is it for me.
Additionally, having a collection piling up on a horizontal surface just becomes an eyesore; it has the OPPOSITE effect; instead of bringing joy from having a curated space, it brings dissatisfaction from having a cluttered and messy space. My brother's housemate had a HUGE Magic the Gathering collection just sitting in a giant messy cluttered pile in his basement. If I personally got into paper magic, I would have a Magic corner where spare cards are archived in aesthetically pleasing binders, and cool cards are displayed.
I am about to get into collecting another thing. I am looking forward to it. It's this new trading card series -- it's not a game, there is no game, it's just collection: "Sacred Legacy", which are trading cards based on the Bible. They are releasing their first set this coming month. I don't actually love their art, but the idea itself is so so interesting to me -- religious art cards -- and I am curious to see if they do a more interesting set. And their marketing is amazing. lol. I am thinking the cards can all go around my altar.
Someday I want to get into the physical paper version of Magic the Gathering and collect on a theme, but it's so darn expensive. I prefer the kind I can play for free on my computer. Even though it's rigged, it's still fun. lol.