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.
You use the word "hobby", but I think this is a unique problem to hobbies involving collections. Personally I stay away from collection hobbies because they inevitably devolve into a binder full of stuff you don't use or enjoy because you already own it, and a rat race to obtain stuff you don't have. That's not my idea of a good time.
Granted, most hobbies are money pits or conversely time sinks, but that's kinda the point. As long as it brings you joy or personal fulfillment.
Or both! I build guitars for fun and, while I've made a few bucks selling some, I'll never break even and I've spent countless hours doing it. Same with working on motorcycles except I have never made a dime doing it.
I guess the bright side is that at least I don't own a boat.