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.
For the CRTs at least that makes sense, because they used to be mass manufactured, everybody had one, and then new tech came out so nobody wanted them anymore. They threw them out or sold them for cheap. Now that that glut of CRTs has cleared out, they're probably relatively rare, and people aren't manufacturing much of them anymore.
I think that probably applies to lots of things.
The same thing applies to game cartridges that the OP was collecting. As time goes on and the pool of interest rises and the number of items are reduced, prices will skyrocket because of supply and demand.