If you want to sell used games, you're going to want to avoid getting ripped off. Growing up I have made my fair share of mistakes selling games back to the a big chain game store for a couple of dollars after paying many times that much for it. To get the most from your game, you need to learn how to sell it through eBay.
First off, you have to make an account with eBay and know that when you're new you are not going to have any feedback. If you want to increase your chances at making sales, try to buy a few things just to make it look like you're trustworthy because that should net you some feedback. It won't be for making sales, but at least you won't seem as new as someone with 0 feedback.
You'll have to decide if you want a flat rate for a game which can take a while to sell, or if you want to put it up in an auction format. With an auction, you give people a certain amount of time to work with to bid on the item, and when it's over the price may not be what you wanted. That's why if you have a rare or expensive game, you're going to want to set a reserve price so that you won't have to sell it if the bids are too low.
If you really want to sell a game at a game store, don't go for the cash option. Try for store credit, and wait until they have a special where you get more on your trade ins if you put the money into store credit. If you go with cash, they are going to give you much less, and that's mainly because they know people are desperate and probably don't know where else to turn.
What about just hanging onto your games because they may be worth a lot more later on? If you have a popular sports title, then it's going to go down in value, but if it's a rare collector's edition of a popular RPG or if it's a game that wasn't well known at first so there aren't a lot of copies, in a few years it could climb in price by a lot. While games are worth a lot more new usually, the ones that really get you a lot of money are those that are rare because not many people were interested when it came out for whatever reason.
Finally, you can try to sell games through Craigslist. Just get together a list of what you have, and then think of prices that are low enough to make people want to come out to pick them up. You're going to want to be sure that you meet with people in a public place to be safe. Always price games you sell this way a tiny bit higher than what you want to make off of them because people always will try to negotiate with you for a lower price.
1 - http://patch.com/california/sanleandro/bp--five-tips-when-you-buy-or-trade-at-gamestop
2 - http://www.makeuseof.com/tag/4-ways-to-sell-your-used-video-games-with-ease/