You're another in a long line of people who have caught on to the abuses going on in the old game market, and in particular with Nintendo stuff, though TG16 wasn't spared either yet so far.
There's nothing singular to blame on it. The trendy shit over the last five years to bring 8/16bit graphics up as an art form has made things a curiosity to the younger and a reminder to the kids of that time. You have those in their 30s/40s now who grew up with the stuff wanting to chase a memory or treat it like comic and card collecting of decades earlier. We have this shitty economy that's not being helped by the scumbag in chiefs policies which turns people even more who are unemployed, underemployed or just underpaid to alternate sources of income to pad the books. Economy or not, the popularity got there enough it pulled the dregs and shitheads of society out in droves who want to pray on the uninformed, impatient and stupid people to rip them off blind playing off desires and memories. As mentioned, you have the popularity of Youtube with all these assholes bragging about their cheap finds videos talking up the hundreds in value for pennies to the dollar, or the poorly acted attention whores like Pat the NES Punk, AVGN and the Mike Mattei flushing out games into the public eye driving demand. And all around that, it's simple economy of supply (which isn't being increased) and demand (which due to all that shit above and more) getting higher, so we all suffer for it.
It happened before if you were a kid in the 80s/90s having comics and baseball cards wrecked. Same stuff with stores opening up all over, people raising prices inching it into the more well offs pockets to the detriment of the child who that stuff was targeted to, and it imploded. The stuff always in time will implode to a point and re-correct itself where the real rarities have value, and the rest get set down back on the bench of reality. But that's another topic, when/if the bubble or whatever you want to call it may or may not pop and how.
All I know is this bullshit drove me largely away from old games and selling off shit I don't want to use anymore, yet due to the awful prices I do keep some stuff had it been still worth $20 I'd ditch. If it wasn't for my recent discovery of the unique games in style and substance on the Famicom I would be a lot less happy about it than I am now. If you know my ebay account or read on NA you'll know I've been parting with a good bit of stuff in the last few years.
Karma you're also right about the 40% (or whatever) bump and a few years ago compared to now how sellers acted. Sure there were always rare games and their prices were justified like Little Samson(Lickle) was an easy $100 even going back 5+ years beyond what the video game price chart website shows, but notice I think it was maybe 3 years back now the game shot up more than double the price in months, under a years time, and if you track it back to ebay you can see this pissing contest of people with the gotta have it mentality and greedy resellers feeding those idiot beasts and it never fell back again after that. All it takes is some tipping point to trigger it and a game can go nuts in a matter of a week. Another one you can look up would be Zombie Nation on NES too which went crazy from cheapo obscurity.