this was done before and they even said the last s-5's they sold were used.
Every single miner bitmain sells is "used". I think they roi most (maybe all) of their gear in house before they sell them.
Bitmain runs their equipment like crazy, I wouldn't be surprised if one or two arrive DOA. Notice how each batch gets cheaper? That's because every batch is based off how much money they make from them (at least that's my theory). S7 B12 looks too good to be true, i'll hold off on them. No ROI expected anyways, so its basically another used s5 from ebay. No profit, and the only difference is that bitmain probably ran it a lot harder than a regular person would.
Batch 8 was really last big change. They have kept same S7 design since then, where they made less chips and more frequency. This change for sure made it more profitable as less chips.
The batches now are sold at lower prices due to difficulty, they seem to follow it. We really don't read a ton of DOA considering amount sold.... so that is the good thing. But it is true when you make a miner in batches you don't sit it in a box new waiting for it to ship out, it goes onto a rack and mines unless sold soon after made. This goes for any company though on making miners no one let's them sit in boxes for long periods of time.