At this stage it's Reminder, User IP Tracker, Credits history and transfer credits, and possibly paid membership.
I think also an important issue is will the dev support such a transfer. If the seller gives the new buyer the files then that could possibly be a nightmare for the developer to support. However if the dev is willing to just issue a new license and let the new buyer download a fresh copy of the plugin would be better.
So the best case is that no files change hands at all between the seller and the buyer, the seller contacts the dev with the buyer oxwall username and then the dev issues a new plugin license issued to that new buyer and deletes the license for the seller.
Although i do not believe that any money should change hands between the seller and the buyer im sure it will but nothing more than the original purchase price (not the current price but the price they purchased it for) And once the transaction is complete i would even require the seller to delete the plugin files entirely (hard to enforce but good people will do so). And then the dev can charge a small transfer fee ($2 or $3 dollars) to the buyer for the devs time used.
If done this way i dont see why it would have any legal issues at all. The main thing is to get everything in writing from both parties so that the dev is covered in the transaction as well as it protects both seller and buyer as well.
Also transferrs per client need to be limited to 1x or 2x a year. This will eliminate people that want to take advantage of the system and become plugin wholesellers which im sure devs dont want.