Deja vu.
He did the same thing to me. Complained about books being inaccurately graded, wanted a partial refund. I only had a few dollars in the books (dollar box finds) and the transaction wasn't for a whole lot of money (less than 50 bucks) so I agreed on a partial instead of a return and full refund. I suggested 10 or 15, he wanted 20. I said ok whatever, just to be done with it. I make a WTB thread, he can't figure out why I blocked him after I didn't want to make an offer on his books sight unseen. Reading his posts in this thread and seeing other people's thoughts about him just my confirmed my suspicions as a character to avoid.
Now 4 confirmed cases.
+5 incoming. i would try to lay out my case but this is it almost exactly. good riddance to bad rubbish.
Yikes! If he claimed that you overgraded, that doesn't bode well for him.