Instructions
Test the games to make sure they work. Even if the games worked the last time you played them, you'll need to test them because games can go bad with age. If you no longer have an Atari system, try to borrow one because the games value will increase if you can guarantee they work. Most Atari 5200 games will work in Atari 2600 and Atari 7800 systems.
Get an instruction manual to go with your Atari 5200 game cartridges. If you have lost the instruction manuals which go with your games, you might want to buy one to increase the value of the game. You can purchase instruction manuals for Atari games at the Atari Guide website (see Resources below).
Figure out how much your games are worth. Search online auction sites for your game titles to get some idea of how much they are selling for. Also search the Internet for Atari game dealers to see how much the games sell for on those sites. While most Atari games typically sell for $5 or less, some rare games sell for more.
Sell your Atari 5200 games on eBay. From eBay, choose "Sell an Item" from the "Sell" menu and follow the instructions. Be sure to mention in the auction listing Atari 5200 games are compatible with Atari 2600 and Atari 7800 systems.