Well, surprisingly, absolutely nothing via ebay. Ebay seemingly don't care and make it hard for you to do anything effective. They have a resolution centre, which resolves nothing. They prevent you from leaving feedback if an item wasn't paid for, and its often impossible to get the buyers full postal address. Luckily that's not where it ends.
First, send you buyer an invoice, and send a copy to yourself (the tick box at the bottom). That should have at least part of the address on it. If not, then ask ebay for it. There is no reason why they should withhold the address of your buyer. Then send a message to the buyer and give them a deadline to make payment. Make this deadline reasonable, e.g. 7 days. Then print out the ebay item page and each message and associated response from the buyer. Keep these safe.
When the 7 days are up and there isn't any payment, then you have two choices.....
1. Take the buyer to court for the full amount.
2. Relist the item and take the buyer to court for your loss.
The first one is simple. Go to moneyclaimonline, fill in the details and get the process rolling. Its not as hard as you may think and its not expensive. You can pursue values up to £5000 in the small claims court and this is where I suggest you begin. When the buyer receives the summons, more often than not, they will just pay and then you simply cancel the case. Those that don't, you will need to either cancel the case and take the loss or continue. I recommend you continue if for no other reason than its the right thing to do.
Remember, the small claims court is just you, them (if they show up) and the Judge and in my experience I've always found the Judges to be fair minded people.
That's the end of this Guide, I hope you found it useful, or at least interesting. The point I wanted to make was don't be out of pocket because buyers think they don't have to pay, I was for years before enough was enough. I could write a whole essay on how to use the court system, and so on, but things change and I'd always be updating the guide. There are loads of guides available online for that, and moneyclaimonline is also a very good resource.