> I've taken to buying used Western Digital Ultrastar drives on Amazon for my NAS.

Same. I bought a bunch of 10TB HGST drives for my home NAS. They were $79 ($95 now) vs $325 new. I ran badblocks for 24H on each and found one dodgy one, which the seller swapped out.

Last year I was buying new old stock 6TB Ultrastar SAS drives from B&H for $90 ea. They were a few years old with 0 hours. None of the drives showed any indication of use, ex: no witness marks. They're in a hotswap NAS.