I don't understand, if you've had payments reversed from time to time, why did you keep taking the risk to trade with him? Because he was paying you back? #followthemoney
Besides the fact that PayPal may decide to close your account due to the number of disputes you've received, you knew such a situation could happen at anytime

didn't you agree to receive a refund of $300? If it is then it's too late to complain dude, it should have been done before he sent $300