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 
What do you think they do when they retire their miners? Throw them away? Maybe the cases and fans, but used boards and chips I'm sure they sell them. I actually wish mine was used, one board died pretty quickly and that's how I know it was new. People think used miners will last less, but the opposite happens with electronic stuff with no moving parts. Well tested is better