Craig's list scam and paypal?

I posted a D3 on craigslist. So far I have 2 responses from individuals that wish to buy the camera for a relative overseas. They offered more money to ship the camera and I won't because its not a verified address.

My question is, if they pay me via paypal, how can they scam me? Can they take back the money somehow? Do they simply contest the transaction with paypal and claim they never got the camera? I use paypal but not enough to know any tricks or deceitful tactics.