Bakewell, my non-payers get an invoice sent after 3 days, then on day 5 I send an email saying 'your parcel is all packed and ready to go I am just waiting to receive your payment'.
On day six, I send another saying 'I am still waiting for payment from you for x that you bought 6 days ago please pay within 24 hours'.
On day seven I point out that they have won an item but I have received no communication or payment and if I don't hear from them in the next 24 hours, I will have no choice but to open a dispute.
That one usually works, however...
If they don't actually pay I then send one last nice mail, saying I really hope everything is ok with them and that they don't have a problem paying etc and once again encoraging them to get in touch and point out that I am happy to cancel and relist rather than dispute if they prefer, after all you never know what is going on in peoples lives and sending them nasty e-mail could be quite unkind imho.
Two days after that I would probably open a dispute.
It pays to be careful not to piss people off, negative fb can be very bad for you as a seller esp when you are starting out.
Personally I would rather miss out on a few quid than mess up my selling potential.
Christ, that was rather long winded.