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

Flash technology used in SSDs has a limited lifetime in terms of writes per location. Although SSDs have the intrinsic feature of wear leveling to attempt to spread out those writes across all locations in the device, eventually one or more locations will wear out and no longer accept data.

As there is no easy way to determine how many write cycles have occurred in an SSD prior to purchase, there is always a risk of getting a device that starts failing the day after one installs it.