It can be a super mixed bag - you can get lucky and end up with a drive that has spent it's entire life sitting on a shelf as a cold spare and was literally only powered up to be wiped so the recycler can say they wiped all the drives, or you can get a drive that has been running well over its MTBF and will fail start throwing SMART pre-fail warnings 30 seconds after your warranty expires

SAS drives are cheaper because the market is smaller. You need SAS hardware to use them, but SATA drives can go basically anywhere.

Hopefully the seller gives some idea of the condition. Usually the ones I've bought have been anywhere between 10k and 40k power-on hours. Much more than that, if they were really cheap, I'd just buy spares.

The other big health factor is start-stop counts. Server drives shouldn't have too many of those. If that number is really high, I'd be concerned about how the drive was used.

I'm running 4 Seagate exos 10tba sas3 drives. They're fast, quiet, and don't use a ton of power.

Now you can get 14 and 16tb drives now for the same price.

Honestly, at this point, I'd rather sell the SAS drives and go back to SATA. I don't need an insane amount of storage, and the controller adds power usage.

I've been running solely on used drives from ebay, I've only ever had one DOA which was refunded without issue and only had one die in service

Hmm, I would only purchase them as a third (or more) layer of redundancy, or maybe for storing things like ripped media that could just be re-ripped (or re-torrented) should the drives fail. I would not trust them for anything important since you have no idea what kind of environment they were in for all those years.