Original trade-in price offered by Gamestop on five games: ~$45
Income from selling said games on eBay instead, not counting shipping: $106.52
Difference between Gamestop and eBay prices: $61.52, or 136% of the trade-in price
I didn't even have to try very hard. eBay has an ISBN database now, so usually I could type one number and have the whole listing completed for me. Retailers had better hope no-one realizes that it's basically the same amount of hassle whether I wait around at the post office or at the store. Not to mention, postal workers rarely ask me if I want to preorder anything.
Although I was tempted by their crafty "stamps" pitch. Next time, USPS. Next time.