Why is it bad to buy used hard drives and SSDs?

I think the biggest problem is that you don't know what you are really getting.  

Mechanical drives do wear down over time, but you can get a solid 5+ years of use out of them. However, SSDs are much worse. They have a specific number of writes and they will start failing. This means... if it was used in data center as cashing or temporary memory... that drive is TOASTED!  

The next problem is that... people who sell the most drives are typically those who use them the most. A small data center, school, company, and so on. They are the ones who have 50+ drives to replace and often times they will sell the old ones. This means you are far more likely to get a drive that has more ware on it than one that doesn't.  

There are some things you can do if you are dead set on buying a used drive.

• Mechanical Drives - Mechanical Drives typically last longer than SSDs and they are normally cheaper so you are out less money in the worse case situation.
• John Doe - Buy from someone local as they are less likely to have heavily used that drive.
• Photo - Ask for a photo of the drive. If the drive was made in 2001 it probably has some extra wear and tear. This also allows you to double check that the drive and post is the same. Some of these companies will have a random receptionist make the ebay post who might not know anything about a harddrive and could unintentionally use wrong information misleading you.