Can you buy and play used PC games?

Yes there are loads of them. The only thing is check that they will run on your computer as I have had problems in the past with my pc being too modern to run very old games but most will work fine. Amazon marketplace has lots and they can be very cheap if there are a lot of one title. Shop around on there first you can usually get a good deal for a product with. I damage that'll work perfect.

Yes, you can buy used PC games and play them on your computer. Another option is http://www.half.ebay.com/, where you can buy and sell used games.

Some games you can but other games you have to have the GEN key. But you can download it or check on the forum of the game for the key (:

It depends. Game companies are using serial numbers that connect to an online account. (Steam, EA online, battle.net) This ensures that only one person plays at a time. So a used serial will not work. If they are selling the account information it may work. Older games that are like 5+ years old will probably work.

legally no PC games are an exception to rules of ownership when you buy a game, you aren't actually buying the game, you are buying a license to USE the game... and this license is not transferable.