He doesn't.
If the buyer did not respond within the deadlines to an Unpaid Item Dispute, the buyer gets a Strike, the seller gets her fees back, and the transaction is null and void.
If the buyer can then persuade the seller to allow him to bid on the relist, and most sellers put deadbeat buyers on their Blocked Bidder List, then a new transaction starts.
Or the seller can invoice the buyer through Paypal without going through eBay.
But the Strike is there as a warning to other sellers that the buyer is a deadbeat, so the rest of us don't have to bother with Unpaid Item Disputes in future.
Well, maybe I'm not a fancy gentleman like you, with your... very fine hat. But I do business. We're here for business.-- Captain Malcolm Reynolds