Buying a used drive, of any kind, you are at the mercy of the sellers ignorance. Just how old is the drive really. Why would someone feel the need to dump a 4Tb Premium drive like the Black, those aren't cheap to buy. How much heat abuse did it suffer. How much wear and tear did it receive. Was it a server drive in a home built NAS where 2 out of 4 drives have already failed. Was it the primary drive for a gamer and has excessive writes at the start of the drive. Was it part of a pc that got burned up by a cheap, bad psu and the owner is ditching everything that seems to work, for now. How much usable life is really left in it before the sectors start going bad.
You don't know. You'll never know. You take it on faith that the prior owner treated the drive like a Princess, when reality is more commonly it's been abused like a red-headed stepchild.
For $50 I'd rather have a brand new, factory warranted WD Blue 1Tb than a used 4TB Black any day unless I knew the prior owner personally and more importantly, knew his PC.