Sunking-
This happens all too often. Out of every 10 auctions I do, 1 does not pay.
On the left hand side of the "My Ebay Page" is a link called "Report an unpaid item"
Simply click here and enter the auction number.
E-bay will send the buyer a notice. This usually gets their attention.
However, if they still fail to pay after something like 3 days, you go to the link below it called "Unpaid item disputes".
This will show you all the unpaid auctions you've reported.
If it's beyond the time frame e-bay sets, you can close the dispute.
The buyer gets a mark on their e-bay record (Not negative feed, thats up to you to leave). If they get a certain number of marks, they get their account suspended.
More importantly, YOU GET A REFUND for your e-bay fees if you close the dispute and they did not meet their buyer obligations.
I hope this helps. If not, e-mail me.