It's a common occurance if you deal in high value items.
Contact Paypal security, the link is on the bottom of the Paypal page and enquire what the status of this issue is. Use the dispute reference number and/or the transaction number and/or the eBay item number.
If Paypal initiated the dispute resolution mechanism, it's typicaly because there is a question as th whether the buyer is in fact the owner of th account used to make the purchase.
If you haven't shipped, don't, do not issue a refund, in fact do absolutely nothing until Paypal has responded to your request for more information.