Every unit I ever received from Bitmain looked brand new. I'm sure they they test them prior to shipping. I don't think a company would stake its reputation on selling premined machines. Furthermore, wouldn't it cost them more down the line in warranty repairs?
Agreed. It's far more likely that they keep a percentage of the machines and run them before or at the point of shipping, so they probably do turn a profit on mining before everyone else's arrive. But primarily they are a hardware company, and based on the volume of sales I doubt they're hurting for cash...