Amazon is quicker, but their seller ratings is stricter. If you have more than 1% of your transactions not rated at A+ then they will really come down on you. Their fees are also pretty high, and there are no pictures.
eBay is more well known for games, but you have to deal with buyers who don't pay (if you go the auction route). It also takes longer for the games to sell (again, if you go the auction route).
Both are good. I've used, and continue to use, both. Post a few games on each and see which you like best.