You do understand that there are different levels of proof required for different transgressions, right? You must, because otherwise you couldn't possibly ban ban-evading accounts, where's the proof? Where's the due process? And yet, I'm sure it happens. How?
It is much easier to tell when someone
is the same person as someone else then it is to tell when they are not the same person anymore. There would be evidence that someone sold their account any time they change their password, move, get a new computer, switch ISP's among a number of other things.
Many transgressions are punished on "more likely than not" basis, such as a 5-yr-old being made to stand in a corner for eating his cookies during class - no trial, no jury.
You need to be fair about any punishment/consequence you impose on someone.