Around here I rarely come across P226 X-Fives and X-Sixes. Will only see an occasional P210 of any sort, even including the new American. Beretta 92s of the uncommon types (90-Two, Vertec, etc.) seldom seem to come up for sale. P7s don't roll in like they once did. Even older pre-poly era S&W pistols are not being 'recycled' back into the used market much these days; rare that anyone brings in a 3rd Gen nowadays, instead regarding them as 'keepers' rather than 'old/outdated'. I definitely don't see as many P38/P1 pistols as I once did. Fact is, I come across more Colt Pythons than any of these other pistols. Used CZ 75s are relatively uncommon around here as well.
In the poly era, Gen1 G17s are harder to find locally; used to come across them regularly but lately not so much anymore. Gen2s are starting to get that way, especially with the G19. Lots of 40S&W Glocks though, and Gen3 and Gen4 guns in all calibers. Used USPs are definitely hard to come by, so my recent purchase of one feels like a real win. P88s were always a rare bird in the used counter, but it surprises me that I seldom see the P99 in any configuration in used form. I've come across more Mk23s than P99s.
On the seldom to never seen modern used firearms list I would have to put the Browning BDM, the Smith and Wesson 845 and any of the Smith and Wesson Night Guards especially the NGs chambered for 10MM auto or 38 Special, then there are the Colt Double Eagles .
I agree the Walther P5c is another one for the list. I would sure like one sitting next to my P5 and P88
But many of the listed firearms in this thread will get a dozen hits just by searching GB or the other auction houses.