Problematic ebay customers?

Have you ever had any problematic ebay customers?
I don’t have much experience selling on ebay, but the little experience I’ve had has been a nightmare.
I sold a camera a few weeks ago, and everything seemed to go well, i received the payment within the same day and had no trouble posting it. A few days later the buyer contacted me and said the camera didn’t really live upto his expectations and asked for a refund!
I refunded, and the camera was sent back.
I’ve now relisted and sold again, but it’s been 6 days since the auction ended and they still haven’t paid! I’ve sent them a polite email asking when then are going to pay, but have had no response.

Has anyone else had similar problems on ebay?
And what should I do?