The legal principle is "right of first sale". In the 9th circuit at least, it can be abridged by a shrink wrap license:

http://en.wikipedia.org/wiki/Vernor_v._Autodesk,_Inc.

Ignoring the legal issue, second-hand purchase of software with license keys, particularly on ebay, is dangerous because those keys may be in use by other people. If the software uses an internet licensing server, it might disable itself because the key is already in use. Buying windows on ebay is a good example.