EBay recently simplified and speeded up the process.
When you open the case as @t_bjarnason explains, the buyer gets 96 hours to pay up.
If he doesn't he gets a Strike that makes it harder to buy on eBay in future and you get your fees back.
A couple of notes;
Most Unpaid Item Disputes are on Auctions. If you use Fixed Price and opt for Immediate Payment Required, the item stays open until somebody actually pays -- or for 30 days after which it will automatically relist.
You can automatically Block customers with Unpaid Item Strikes in your Seller Preferences/ Buyer Requirements.
