When I sold my MBP 13" I simply created a user account named "Seller" and then deleted my old account along with all the associated files. Any logins or passwords I used were in my browser profiles and my keychains were erased with my old account.
Sure, there are "ghosts" of info that can be extracted, but unless you're selling your Mini to the C.I.A., they're going to be pretty hard to get to, and the new owner will most likely begin putting files on that will overwrite the empty space almost immediately anyway, so they'll be even more inaccessible.
In reality, you actually never "wiped" the files from the
drive. Deleting or trashing them only removes their
locations on the drive from a table. Anyone with a little
saavy can purchase a file recovery program and will be able
to recover almost all that data and with enough time know
what it all is.
The ONLY way you should sell your computer to a stranger is
to completely reformat the drive and in the options select a
single pass of writing zeroes to the drive. This will effectively
reset all the bits on the drive to zero. This will be sufficient
unless your giving drive to some CSI with expensive disk scanning
equipment. After that, reinstall the OS that came with the computer
and set up a bogus user.