> Such abuse includes, but is not limited to, buying or selling one’s seat in a class, holding seats for another student, or otherwise registering for a section that one has no intention of taking."

This bit is important.

At a glance, stubhub/ticketmaster/etc are pretty benign services that fulfil a pretty natural need for event tickets, but we've all seen the damage they can cause. There's a very real risk that OP's service could become a ticketmaster for UW classes, with all the perverse incentives that entails. Their reaction was probably excessive, but, from this perspective, understandable.