hahaha this is a great situation (from my point of view as a noninvolved person 🙂 )
What you should really do:
Put your ebay username as the return address
Don't include your real name in correspondance
Send him his hentai, hope he doesn't notice the @purdue.edu address, and don't look him in the eye during class
What would be funny to do, and thus I recommend (once again as a noninvolved person who can experience no negative reprecussions):
Be sure your name is VERY clear on the package
When in class, mention you sell "umm...items" on ebay. Mention your username. If asked to clarify about the items, just say they're in the adult section.
When you go in to take the final, draw a very detailed hentai scene all over the test, with your ebay username on the Name field, and write "A+" off to the side with an arrow pointing at that direction (or a tentacle, whatever strikes you fancy ya perv 😉).