It depends on the method of payment. If it's a Buy Now item I would make it so that the item was paid for by Paypal straight away. If it's an ordinary auction you can put in the listing that you require payment within 3 or 7 days (your choice).
A bit trickier with other types of payment. For example, how do you know that a letter containing a cheque or postal order hasn't been delayed by the PO? Then there's the cash on collection option.
What I'm saying here is that it isn't cut and dried and that more info is needed before we can give a worthwhile answer.
