Instructions
Take them to a used game store such as Play and Trade or Gamestop. These stores specialize in buying and reselling used games and will rarely reject buying your game as long as it's in good shape and you have the box and instruction manual. However, they typically will only buy a game back for a third or so of what they resell it for, and many stores will not give back cash, preferring to pay you in store credit.
Create an online listing using a website such as eBay or Craigslist to sell your games. Remember to include a photograph of the actual game being sold and all of the components that come with it, including the case and instructions. If you're selling only the game disc, then you should charge a smaller price for it as opposed to the price it would have if it included all accessories.
Sell them at a garage sale. People often scan garage sales just for old electronics, and your older PS2 games should fit the bill, nicely.