I've left negative feedback before and, truthfully, it isn't something I care to do over music. If it was a car, or something more valuable, I would.
In this case, however, if jazzhound has payed a nice dollar to get this LP (what was it by the way?), and it's not up too par...he deserves to know!! Dishonestly should not be tolerated in terms of LPs. That said, if the seller has offered you a refund, what else can be done?
It's a tricky situation, but, ultimately, this man deserves to get his money back. Hopefully, it works out for you.