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.