The key things to watch for when bidding on GBA games:
1) Seller from Hong Kong, Korea, China or other Asian nation -> almost guaranteed pirated
2) Seller has only been registered for a couple of months, but has tons of feedback already from selling GBA games (usually means that they jump from account to account as they get shut down for selling pirated games)
3) Selling 'new' games, but says the box comes flattened to save on shipping
4) Is selling 'used' games, saying they didn't like it, but when you check their other auctions, you find they're selling dozens of copies of the same game (I remember about a week after Pokemon Fire Red/Leaf Green came out, a guy was selling DOZENS of both, and all his auctions said he beat the game and was selling it to get rid of it. If he actually beat 50+ copies of Pokemon in a week, he has problems...
5) Lots of spelling and grammar errors that make the auction sound like it was put up by a non-native English speaker
Assuming that the auction passes the above tests, its _probably_ legitimate. Maybe. Good luck
