People list lots of stuff on eBay as FPONW because they either can't test it (e.g. don't have a PC with a PS/2 connection), or they don't want to bother testing it.
This doesn't mean it doesn't work. In fact, except where the seller has specifically said they don't work, most M's listed as FPONW probably work just fine. I've bought a couple myself, because they otherwise looked good, and found they had no problems and that I'd saved some dough.
You take your chances, though. "FPONW" is a disclaimer. If it's broken, you're stuck with it.
That said, I'd steer clear of M's this dirty. They've obviously been left outdoors or in some unusually dirty place. M's don't don't usually get that grimy even after 20+ years in an office. I don't think it's worth taking the chance they're something mechanically wrong with them and having to spend so much extra time cleaning them up.
That's how I see it, anyway.