I think for me it's retro games, specifically. I used to have been in the used video games market for 5 years from 2008 to 2012. My goal was to construct a personal video game collection, physical copies of games I personally enjoyed growing up.
I was registered on a game trading site which served as the base of my business, I've made rounds of thrift store hopping and any used games market I could find locally. I've struck amazingly good deals and I might've had luck on my side a few times (for example, a guy on that game trading site gave me a free copy of Super Metroid that I got to choose for a minor mistake he felt he needed to honor.)
And I felt like I was incredibly close to completing my personal collection until 2012, I ran into some dumb drama with my sister and ex girlfriend back then. They racked up the cable bill in my name that I was trying to cancel and they wouldn't let me cancel it until I turned in all equipment. And I was jobless at the time too, having lost my job. So I needed to sell some things and sure enough, had to sacrifice my entire collection at the time that I spent 5 long years building.
I never recovered since and this was during the golden period where it was still fairly fun to collect and everybody wasn't pretending to be a pawn shop.
I would try continuing what collection of games I've tried to build, through Steam but it wasn't the same. Nowadays, the used video games market has turned into just a platform full of resellers, pawn brokers and stingy greedy collectors.
I find it very cheapening that people treat games like they're just tools of trade. They mean nothing and they're treated like nothing except to make a quick buck, however possible.
It's only worsened thanks to Goodwill and similar thrift stores, getting in on it where everyone pays too much attention as to what the prices go for on EBay and VGPC.
And we have WATA involved that hasn't made things better. Thanks for shitting on an honest hobby, assholes.
Anything 'collectible' has become a no-go for me. Simply put, anything that is collectible has turned into a business, with more or less artificially created scarcity and with entire businesses thriving on people gullibility, FOMO and with their obsession with making a quick and easy buck. There is no way I want to participate in that scam.
On a more personal note and experience:
I was a book and comics collector for many decades myself. I started as a book-lover teen and and then as a student, selling used books and comics as a way to earn money and then pay for College. It was a lot of fun, back then. Like really. And exciting too.
Then, as a young adult earning more than a decent living, I started spending serious money in rare and original editions, this time without any idea to make money out of it. It was just my hobby and I earned enough to not worry, and it was still fun and exciting too. At least, it was fun in the beginning.
Still, a couple decades ago I donated my entire library (3k+ books) to a charity because... Well... Money was king everywhere. And what I used to love was now making me feel so sad.
Sure, I should be happy as I had accumulated a small fortune on my shelves but I did not enjoy it the slightest. People (like myself) were not giving a crap anymore about content or the authors they once genuinely admired, they only worried about the books value. Realizing I was that kind of person myself made me feel very ashamed of myself. I wanted to get back to what really matter, the content, the text, the art. and not their fucking retail value as collectibles.
Getting rid of my entire library (save for very few books, not even rare) was liberating and, so many years later, still feels like the best decision ever. I don't care the slightest about those books potential resale value anymore. I barely own any books myself nowadays, I read most of them from the public library, and I've rediscovered the unadulterated and incomparable pleasure of enjoying reading.
This is partially why I started collecting vinyl. Nothing I buy will ever have a major resale value, it’s there to be enjoyed, because I admire the artist, or found the album to be a masterpiece. They remind me of times in my life I want to revisit, and putting on a record can take me there.