I'm sure there are a few badge sellers who know they are selling what is deliberately stolen property - but that is true of people selling anything, not just pins. I doubt that most pin sellers go creeping around campuses late at night, snatching pins from bedside tables just so they can make a buck.
I would think that particular argument would apply more to, say, a girl my age who signed an agreement saying if she quit she would return her badge. (I signed one when I joined.) She then quits but sells the badge on eBay. She has violated a written contact. However, this is not the majority of badge sales by any means.