If you buy an Oculus rift from Ebay or if you buy it used, does the unit still have a warranty? Trying to get an official response, I know Oculus is pretty good about preventing scalping, this would be a pretty good deterrent.
In the UK a warranty of 12 months is a requirement but I'm not sure on transfer of that warranty between end users, perhaps that would be a nice weasel clause. Still, it'll be a small price to pay to honour an end user warranty as long as it is less than 12 months since the original purchase for a good user experience.