If you are an e-Bay seller, there are a few handy eBay features you can use to avoid malicious bidding and unpaid items.

Of course, the best way to avoid unpaid items is to offer PayPal, describe your shipping terms clearly, and communicate with your buyer early and often.  Many unpaid items are due to an oversight by the buyer or an honest misunderstandings and can be resolved amicably.

But there are some situations where there is a truly bad or malicious buyer -- someone who is intent on "buying" a large number of items and never paying for them.  What can you do in this case?

Buyer Requirements
One of eBay's answer is Buyer Requirements (available from account Preferences).  It allows you to restrict the type of buyer who can bid on your auction.  For example, if you only ship in the US and get UPIs from foreign bidders, you can block bidders who live outside of your shipping region.  You can also block users who have a risky profile (negative feedback or previous UPI strikes).  If you sell merchandise that is high price, it may make sense to limit the # of items that any buyer can buy from you in a 10 day period.  The nice thing about Buyer Requirements is that you can decide how restrictive you want to be.  Also, you have an exemption list that enables you to certain trusted buyers through. 

Immediate Payment
For fixed price and BIN items, if you want to totally eliminate UPI, you can require immediate payment on your items.  That way, the buyer must actually pay using Paypal before the item is considered sold.

Report it to eBay
If your buyer appears to be a normal user who has not paid after at least 7 days, use the Unpaid Item Dispute Process.

If your buyer appears to be a malicious bidder (showing a pattern of very rapidly bidding on a large number of items without the intention of paying), you can report the user immediately for investigation by eBay.

Good luck and happy selling!