>It always interests me that people sell old versions of software without realizing they may not legally be able to do so.

Has there ever been a case of a software company going after someone who sells an unused copy of a software product? I doubt it. Companies that want to prevent it on a specific product add activation (Office, Photoshop, Lightroom, Windows, etc.). What matters is when people make copies of software and then sells it to multiple people - same as music on MP3, etc.