ive been buying and selling on ebay for almost 2 years, and have brougth a few gba games in the time, and most all have been fake, you can tell a fake gba game by-

• the box is glossy
• the game cart feels odd compaired with a offical nintendo cart
• if the seller is selling games like pokemon chaos black, try finding that one on the nintendo website!
• if they wont offer you a refund if you brought the item and its 'not what you expected'
• they offer your a replacment if the game wont save, but if it was a original nintendo game, there wouldnt be a problem, because it would save
• 9 times out of 10 the game will come from asia

but if you get fooled by there low prices, because the game is 29.99 in the shops, and on here is 8, these are the things to look for when getting the game in the post-

• the box is flat packed
• the cart sticker is very smooth
• the cart feels tight when putting it in the gba
• the game wont save
• the screen shots on the back are fuzzy

i go by the saying now, if its too good to be true it normal is

to be on the safe side, i would email the seller first just to be sure

hopefully ebay will catch on to this sooner or later

hope you find this useful :)