Well that's kinda dumb, the manufacturer is always Sony so you should unlock it regardless, its your device you do what you want with it.rule of thumb has always been:
phone is bought from sony = bootloader can be unlocked
phone is bought from a phone company = bootloader cannot be unlocked
either way, you can still customize pretty much anything except the kernel. that means no cyanogenmod, but stock based images are way better IMHO.
unlocking the bootloader wipes the drm keys, but you can backup the drm keys before unlocking the bootloader and restore them afterwards.
and to be honest, those drm keys aren't used for anything useful anyways.
So even if you want to buy it used, you're basically gambling to see if you get an unlockable one or a locked one.