I just went across a fleamarket and saw tons of GB, GBC and SNES games but I didn't know which ones would be worth it. I think it'd be good to get a "these are worth it if you see them" list together. Please don't mention the obvious Nintendo games, they'll just clutter up the list and everyone knows them anyway. NES games weren't sold at all, only SNES, GB, GBC, N64 and modern consoles.
Out of my old games (at least the ones I can find and/or remember) I'd say these would be worth buying on a fleamarket:
GB:
Nemesis 1 and 2
Parodius
R-Type 1 and 2
Megaman (but beware, it's effing hard compared to the console games)
Gargoyle's Quest (probably, haven't played it very far but looks good so far)
SNES:
Tiny Toons: Buster Busts Loose
Megaman X
Super Bomberman 2
I've seen some games quite often on the market and wondered if they're any good:
Axelay (SNES)
Spectre (SNES)
Exkaliber 2sumthing (SNES)
Shaq-Fu just kidding, I know about that one
Dropzone and Super Dropzone (GB and SNES, respectively) (Is that the game where you're playing an astronaut-looking dude who can use grappling hooks and a gun in a sidescrolling jump and run?)