If you're talking about secondhand software, I've seen folks sell their old copies to (online and offline)secondhand retailers without any apparent legal trouble. The only things the law(in the US) bans on this subject as far as I know are some clear no-no's such as either buying one original copy of the software and making copies to sell or give away or buying a legitimate copy, making a copy, and returning or selling the original. In most cases, as long as you do not do either of those things, you don't have to worry about selling an old copy. Some companies will try to restrict what you can do with your software concerning redistribution so you might want to check the legal agreement carefully before you take action. Usually, what they want you to do regarding reselling the software will be clear.