It's not all on the rise, if you look back over ten years.
I used to work in London's biggest import game store through 99 - 2001, right in the heart of the city. We had two floors of import gaming and it was packed to the rafters with people all day long. At that point everything was relatively expensive: Used AES was very dear and I traded a kidney for Mark of The Wolves.
PCE stuff was also pretty expensive, but it was Saturn that I noticed rising long after I left the place. This is all pre-internet boom of course, but it was a great place to witness the genesis of a serious collector market.
These days Saturn has fallen compared to ten years ago, Megadrive has fallen too (my first visit to Japan had me ignoring titles like Vampire Killer and Hard Corps because they were too expensive.) PC Engine has fallen and Dreamcast is a non-mover for the most part. MVS and AES has dropped too (I remember when Pulstar was exorbitant). PCB's fluctuate, but Cave stuff has definitely decreased on the whole.
Nowadays it seems to be Nintendo goods on the increase, probably because decent cardboard is becoming hard to find thirty years later. But Akihabara is a joke these days - I've seen cheaper stuff on ebay than I saw when I was there last. They have premium stuff at prices that don't constitute any kind of saving versus forums or fair YHJ auctions, and I'm sure that's having a knock on effect for Gaijin based ebay sellers that use Akiba as a base for goods flipping.
-Ebay is a mess... their shit policies for sellers will even contribute to higher prices since they are taking on WAY more risk now than 10 years ago
-Lack of character in sellers (many not in the hobby out for a quick buck... some just don't know the value of repeat business)
-Shift in taste/availability
-A younger common denominator in the hobby. Face it, we're not getting any younger.
Agree with all of these things. I'm really waiting for that stupidly strong yen to finally nosedive (it's gotta happen soon) to see some kind of marketplace apocalypse. I think the entire playing field would shift.
Always outnumbered, never outgunned.