This is important, because it's the only explicit reference to "trading open spots" they've made.
The Registration Abuse policy covers access to the registration system for:
* 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.
It's unclear if HuskySwap actually violated this policy, given that it never actually accessed the registration system and no students used it in conjunction with the registration system of the school.