I don't know how you manage to keep being confused about this whole deal. When you buy a disk, you don't care what's in the empty space. You format it as an encrypted disk by writing a few sectors. When you want to throw it out, you delete the keys that decrypt what you wrote over the life of the disk, and perhaps wipe the sectors containing encryption metadata. What is unclear about this?

Besides, when the disk is configured as encrypted, the junk in the empty sectors can't be read by decrypting it. Unless it's software encryption and you bypass it by poking the disk directly, you can't read anything from those sectors. Which, again, contain either noise, or what the possible previous owner has written there.