OP,

"I've had a very odd experience with a person getting revenge for me giving them warranted neg f/back 4 years ago"

While you thought it was warranted (all neg givers always do - otherwise they wouldn't give it) - the lady obviously didn't think it was warranted.

I'm not a seller - but I think it stands to reason that those who neg people and also sell  are always left wide open for this sort of thing to happen.

Not that it makes it right - but the possibility would surely always have to be there - if it's all done with the same account.