I'm new here so take it for what it's worth, but my December batch S9 was as clean as a whistle and looked brand new. My January batch will be here tomorrow (go figure) so I'll report on that one. But I would offer up that the theory of Bitmain selling used units is pure conjecture. Just my observation and opinion 
Unless somebody comes up with a proof that Bitmain is indeed selling used units, I would assume otherwise. Common sense and industrial practice would point otherwise. Complexity of moving used units back to factory, packing them, updating shipping inventory, SKU's, employees doing this versus something else. etc .... would cost more versus the potential profit.