I've heard a bunch of people say it is, and I get it. You buy Mario Galaxy for $20-$30. You nearly break you wrist spin jumping over everything, beat it, then you want the sequel. You head to Gamestop and trade in Mario Galaxy and find your game is only worth $7. Wait, only $7? But I paid so much more D:< The thing people don't understand is you aren't returning what you bought. You're returning a disc with a game. What you bought was the game experience. Nintendo sold you hours of entertainment and italian space plumber fun. The game disc itself is worth $7 to Gamestop, they then sell that, and experience to yet another gamer for a discounted price. If you were able to resell the game for around full value, you basically bought the experience for free. As for Amazon, they have better selling deals than Gamestop. That doesn't mean you're getting screwed over. It just means you're dealing with a business who needs profit margins.
In short, Gamestop is a business, You're either helping the company who bought thousands of games from the designers of your favorite Game, or giving/getting $40-$50 to somone who only bought 1 copy.