Is it possible that your seller is new and inexperienced? If so, the funds are being held by paypal as insurance for his/her good performance. (All sellers must pass this hurdle when they begin to sell on ebay.) That, however, is not your problem as the buyer. The seller is supposed to ship as soon as you pay, regardless of when paypal releases the funds 21 days later.
Tell your seller one last time to look for the payment in his Account Activity on Paypal. It will say Pending. If it is passed his stated handling time in the ebay listing, and still nothing has been shipped, go to the paypal Resolution Centre and open an Item Not Received Claim as soon as they will let you.
