I hate to endorse these things...but I feel like people need to know so they can AVOID them.
ioffer.com is OVERFLOWING with GBA bootlegs, multicarts, famiclones and knockoffs of all kinds. You'll see poorly hacked pokemon roms set to cartridge format, and pretty much every GBA game sold there is a fake. Don't go there looking for anything legit. :\
And in fact, a lot of these sellers have started branching out to ebay, selling bootlegs there by keeping the descriptions under the radar.
Things to look out for:
Crappy/off spec labels.
Lack of a box/manual
Mention that "battery might not work."
Mention that "game cannot be saved."
mentioning "China, Chinese import, Chinese version"
Presence of a Yu-gi-oh character on the front of a Pokemon game.
Don't get hosed!