Wow, this is difficult. normally, I'm in the group that says all of them (and I haven't sold a game since the Saturn days, so I think I could say that).
Though, I fully buy into everyone has a price. Sure, for enough money, I'd sell anything that I own (video game wise), but here are the things that I'd need way over going rate to unload:
Nights (Saturn)
Panzer Dragoon (Saturn)
Panzer Dragoon Zwei (Saturn)
Panzer Dragoon Orta (Xbox)
Morrowind GOTY (Xbox)
Final Fantasy X (PS2)
Final Fantasy ViI (PSX) (only because I have the non greatest hits version)
Mike Tyson's PunchOut (NES)
Super Mario World II (Snes)
Gee, more games than I really wanted to list. The sad part is, I can think of another 10 games off the top of my head that I want to put on the list, but after awhile, it'll just make the list too long to be meaningful.