Hello!
I'm trying to sell some PC games over ebay or other classifieds but I'm concerned because I know the serial key will work only once, so I'm wondering if I can unlink the serial key from the games specially EA games, like BF 2142, NFS Undercover, Pro Street which are games that require the serial key to be played.

Do you have any suggestions or advice if there is a way to do that, Thanks in advance.

Typically they only allow ONE instance to be online at any given time. So as long as you do not play the game anymore (since you sold the disc), then all should be well. Or you can just uninstall them and not worry about it.