PayPal Says Man Owes Nearly $300 For Dispute That He Won

Kentaro already went through a dispute resolution with PayPal for an HTC Droid Eris he sold on eBay. He says the reason for the dispute no longer exists, and anyway, he won and that was supposed to be the end of it. But now he owes $287, according to PayPal.

A few months ago I sold a brand new HTC Droid Eris on Ebay. The buyer had trouble activating it, so for two weeks I was on the phone with him helping him out and we finally got it to work. Great! Another satisfactory buyer and transaction through ebay. Now what happened was prior to us figuring out the activation issue, he filed a dispute. I responded to the dispute by explaining our situation and noting that the phone is activated and there is no longer a problem.

I won the PayPal dispute, and the case was closed.
Come two months after, I get an email saying I owe PayPal money. Not sure what was going on since I hadn’t made any transactions for weeks, I log into the account. Apparently, the purchaser who had used his credit card for this transaction had his cc company do a chargeback, and PayPal decided it was my fault.

When I won my dispute, PayPal told me that any further expenses would be paid for by them because they “believe I shouldn’t have to worry any more as I already won the dispute.” It seems obvious that this is not the case, and all emails/phone calls to PayPal have gotten me nowhere for weeks, and $287.37 is a lot of money for me to cough up. What should I do? Thank you for any help in advance!

Any advice?