Why not to sell you HDD or SSD with your computer

[WTFX]

I use bitlocker to encrypt the disk, delete the key and empty the data before selling to avoid data loss

You mean create data loss. Very much so too. Sounds reasonably effective. Might format it a few times afterwards for good measures.. Just in case you think "they" may have backdoors in the encryption. I thought I read recently also that Bitlocker is somewhat backdoored/flawed.. might have been only in default configuration though.