After I have paid someone for something on ebay (via Paypal) how do I go about getting the money back?

I remember reading a thread on here (searched but can't find it ) where someone got screwed cos they were paid with paypal then a few days after they sent it out the money was taken back buy the buyer.

Well some cock has tried to fuck me over on ebay and sent me duffed goods, so I want to clim the money back - how do I do it?