I don't think the vast majority of these are anything other than willful self-deception on the part of the seller. They bought the bike years ago, remembered it as a "top of the line French racing bicycle", maybe did some online research (but not enough). Maybe a shop told them it was a PX. They figure they've got a gold mine and put it on Craigslist.

I see at least a couple a year. The last one was indistinct enough and promising enough I actually went to look at it. I brought along a PX, and after ascertaining the bike was indeed the ubiquitous UO, showed them the differences. They adjusted their ad rather quickly-- there was no real intent to decieve.

UOs are identified by their lugs, braze ons, "tube special allegre Peugeot" sticker, seatpost diameter (usually 25.4, although many UO posts are smaller and shimed to fit the bike; PX posts range a bit from 26.2-26.6), steel rims, cottered cranks, and several things I'm probably forgetting just now. If that sounds like a long list, it is, but they're all things that may not be readily apparent to a person who doesn't really like old bikes like we do.

I thus generally cut UO sellers who identify their wares as PXs some slack. With a few exceptions, they're simply poorly informed. Those few exceptions tend to be outright fraudsters who become belligerent when challenged. They mistake facts for a pissing contest for some reason.