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.
Magic the Gathering.
I played religiously in high school. Competed in tournaments every weekend. But around Stronghold, they just started churning out expansion after expansion after expansion, and even placing high in most tournaments, i just couldn't keep up with the older guys that had incomes. So, i quit.
When Commander came out, i tried to play again, but it just felt different. It was more about who had the coolest mat, or who had the newest combo. It felt monetized, and i didn't like it. The final straw for me was when i found out they did a promo expansion with My Little Pony... If you like MLP, cool. More power to you. But i HATE MLP with a rage that could scour all life from existence. My little sister was obsessed, and that meant it was in my face for a decade at least. Can't stand that shit, and wizards going cash grab during the height of the bronie movement just killed the last of my good will.
Man, if you thought 1998 had too many expansions...
Wizards of the Coast was bought by Hasbro in 1999, but only in the last five years or so have they really seemed to open the floodgates with all the Hasbro and other IP crossovers, multiple versions of every card, etc. It's not surprising since other toy sales seem to be in a slump, but it's wild that Magic is keeping one of the world's largest toy companies in the black.