Let's get this started with an easy one, shall we?
Gorf: https://pinside.com/pinball/forum/topic/bad-seller-alert-gorf
Although he's already banned, he will likely try to come back to Pinside under another name and he should not be forgotten just because he's banned.
I also nominate treasurecove: https://pinside.com/pinball/forum/topic/buyer-beware