I bought a used Spore from Ebay and prior to that tried installing my nephew's game and had problems. The issue is registration keys. On Halo CE, I was able to switch disks on a computer. As long as we were not playing at the same time, we could play the same game loaded from the same disk on different computers. Despite if you are able to play without the disk, you need to be aware that if you registered the game, you will need to close your account for that particular game, prior to selling the game.

If you want to buy a used PC game, you are in for potential problems, especially after 2010 or so. I was able to get a new key from EA, but it took a lot of e-mails and an hour on the phone to get it, that took at least two days. The first call had a half hour expected wait time, where I hung up. Just because I had the registration key, does not mean I had full access. In this case with Spore, I was able to play, but could not do any online activity or register the game.

It is suggested you buy new, download the digital version or only take a PC game from the original owner that you have contact with. (Mine was bought from Goodwill in CA, where the original owner likely dropped it off and the store had no record of who that person was.)

Also, you need to worry about software and hardware compatability. You buy a game that you can't return could be a headache, instead of cheap fun you intended on.