Sorry if im coming off rude, but you are not getting screwed over. This is how trading games work.
If you trade with a friend, he will most likely realize that he doesnt want to piss you off, so he wants to make a good deal with you, so that everyone wins. The focus is not on money, and is just to make a good compromise. Thats where you could probably get you $5 and $18 dollars back.
The problem is, stores take many of these things into consideration.
1. They dont know you, and could care less if you went bankrupt tomorrow. Its not cruel, its just that if you dont like their prices, dont get there.
2. The idea of selling a game is that you dont want it anymore, while someone else does. When I play a game to death and dont want to play it anymore, Ill take whatever money I can, and it is the same for many other people. So while you can sell that game for whatever you can get, the store can jack up the price in order to make profit. Thats not mean or selfish, thats how business works.
3.Imagine if every kid with a ps2 decided to sell all their games to buy ps3s, and got good money for all of them. Gamestop would go fucking bankrupt instantly. You also need to keep in mind that it takes time to sell, and if they are stocked full with the same copies of Madden 2003 and Max Payne, then they arent going to sell them at all. Just be glad that they didnt just tell you to piss off, and that they had too many copies of the games their buying in.
4. Gamespot is not your local blockbuster. You cant just buy a game and expect even close to half your money back. If every customer traded in their used games back in, then the gaming companies that provide the games you love will go out of business, as stores wont need to restock.
I work a mom and pop's type of game store, and if we didnt buy old used games for less than 20% of what we sold them for (new releases would be much more though), then we wouldnt trade at all. There would be absolutely no profit of having stocks full of repeated games, and just giving out free money to the kids who decided to give up gaming in exchange for shitloads of cash. World doesnt work like that. If you want good money for your stuff, either sell it to a friend, or go on ebay. That way, you dont go straight from seller to consumer, without any middle men.