You drafted a bill of sale supposedly and this document says that the tires are sold "as-is" so I don't see how he's in the wrong here. If he offered 100$ plus a pulley of unknown value, I would believe you'd come close to the $300 you want. He doesn't have to give you a dime based on the document you drew up.
With that said, it sucks something awful this happened to you, but look at the guys feedback. If he was out there selling bad tires or whatever, there'd be more than one person blowing him up out there.
E90 LCI N54 6AT