Achow (I think that was their name) had a website where you could enter any users UID number and it would tell you what the market value of that account was (there were even modifiers such as green trust feedback Vs Default Trust which would increase the asking price)...

It was a double edge sword as those selling accounts would say it's value and trust feedback scores and we could pretty well work out which account was about to be sold with confirmation coming in the form of a change or email.

Then accounts were sold with a wallet address signature thrown in via account sellers such as tomatocage who started the stake your signature thread with a few of the first few pages all accounts he then sold with those wallet address - if there were any doubt the accused would just sign a message.