Go to your Seller Preferences and join the majority of sellers who have set an automatic Block against bidders who have two or more Unpaid Item Strikes in the past 12 months.
If those bidders get 'too many' Strikes* , eBay will close their buying account.
When you get an unpaid transaction, you should take action too. Go to the Resolution Centre and open the Unpaid Item Dispute that will award the bidder that Strike. You will get your selling fees back and can relist without difficulty. The buyer cannot then leave you negative feedback, either.
This helps all of us.
Negative feedback? Not only does it reflect as much on the leaver as the receiver, but it is totally useless for all us sellers who don't use auctions. We don't see who is buying until they have bought.
But we can prevent those purchases by using the Strikes. It's automatic. We don't need to even think about it.
*best guesses are four - with the first being removable, since anyone can make a mistake and learn from it.