I have some friends that do business in Shenzhen and Hong Kong. They've warned me to steer clear of such grey market stuff for two reasons. First is the obvious no warranty if you are buying a white box lens on ebay from someone who isn't an authroized reseller. What was more interesting to me was they allege that people raid the dumpsters of factories looking to get items that did not pass QC. For some things like shoes or handbags NBD. But for lenses, and computer items it's concerning. (my buddy and his wife run a flash memory business so they deal directly with a lot of companies over there).
He also said some companies have shady managers that are underpaid or just greedy who will directly sell batches of the failed items to folks. I really believe that, knowing a lot of these places have to just eat the cost of failed items otherwise, often being subcontractors.