Another (non-photo) site I frequent has a thriving community of people selling off software and plugins that they bought but no longer want or need. This makes me wonder if there's something unique to graphics software licensing and plugin licensing that precludes easy transfer.

Have you ever sold or bought a transferred license?

If you're a developer, what's your opinion of this? Do you allow or even encourage it?

Intersting, I would have thought that was illegal.

here in germany a private person can sell software you bought (also OEM) as much as you like :)