Wait, the guy paid you the whole $2,100 though, right? Sounds like the $600 he stiffed you on was the last part of the $4,800 you bid, not part of the cash offer.

Saw a thread about this on Reddit last year where a guy in Ontario tried the same thing and got hit with back taxes plus interest. The assessor always finds out eventually, might as well get ahead of it.

Have you thought about whether the comebacks just moved to a different failure point instead of going away? Meaning your notebook only catches the stuff that comes back to you, not the stuff people quietly take to another shop or just live with.