Whilst sellers can open one 2 days after their listing ended,I give my buyers 5 days within which to pay. It seems more reasonable and hopefully prevents them from feeling harrassed into paying (and subsequently leaving poor feedback). It's a personal call though, you may wish to open one sooner.
The non-payer will get a strike against their account, and sellers can set their Buyer Requirements to block bidders with 2 or more strikes.
They also won't be able to leave feedback for the transaction, which is always a bonus when they turn out to be flakey.
Add any non-payer to your Blocked Bidders List. You don't want them coming back:
