On Day Three after purchase, send a polite note to the bidder asking if he needs help with Paypal. That's just an excuse to get in touch, you really can't do much for him.
If there is no useful response, open the Unpaid Item Dispute on Day Four (96 hours after purchase, to be exact).
If the buyer does not pay up, you can close the Dispute in four days, get your fees back, and relist. No feedback can be left by either party.