I'll throw a couple of *Guesses* out there...
(1). I think that, in a way, it makes sense that you would encounter games from older & more obscure systems more often than PS1 games because PS1 games are still more conveniently *Playable*.
What I mean by this is that PS2's and PS3's are all backwards-compatible & capable of playing PS1 games as well.
In the case of older, out-of-date, no longer for sale hardware...playing games on them would require hardware that is much less common, harder to find, harder to repair, find parts for, maintain, and even find *Others* capable of & willing to work on them for you.
Thus, "games that are more difficult to find, maintain, and repair the required hardware to play them on" = "the games that people will sell quicker than their games that can still easily be played on the more modern consoles that are easily available...and quite possibly, already in their homes".
(2). I would think (and hope) that as more & more people become more "Internet-Savvy" (in realizing how to quickly & easily ascertain the value of the (PS1) games that they may be considering selling)...that they're going to be smart enough to realize that garage sales, flea markets, etc. are *NOT* the places to put these items up for sale as they can get much better & higher prices for their games by listing them for sale on Amazon, e-bay, (hell...even Craig's List).
-Like I said - just some guesses...