Summarize the following:
The employee will either ask to see identification (some retailers require you be over 18 to sell games) or begin scanning your games to see how much they are worth. They may also ask you to fill out your information, such as your name, address, phone number, and email, for their records. The value of a game is determined by how rare and popular it is, how many copies the retailer already has, the physical quality of the disc or cartridge, and the game system that it is for. Some stores offer a larger amount in store credit than the money they would normally give you. If you shop at the store often and plan to trade in games regularly, it might be worth it to accept the store credit.

Summary:
Bring your used games to a retailer that accepts them. Go to the checkout counter and tell an employee you want to sell your games. Accept the trade-in value.