If the data was always encrypted, then simply discarding the keys effectively means the drive is left filled with random data. Also, NVMe drives can be sent the sanitize command which can erase/overwrite the data across the entire physical drive rather than just what's mapped into the logical view. I believe there's SATA commands to perform similar actions.

> t's not safe to sell SSDs is it?

Bitlocker (or anything comparable) makes it safe or ATA Secure Erase if you can issue it (not usable for the system drive most of the times) and check it afterwards.

> And even if it were, who would buy a used SSD with unknown

it doesn't worth it for $30 drive, for the multi-TB ones it's quite common, especially for the ssrver grade ones (look for the PM1723/PM1733)