How much you trying to sell it for?? It could be a scam, did you look his info up....did you recieve payment and then shipped it out>> It could be, but it may be a true person paying and not trying to scam you, but be very careful with paypal, I've had issues in the past.
You can usually tell if someone is trying to scam you. If you are selling on eBay, do NOT accept bidders that have zero, negative or less than five positive feedbacks. I sell and buy regularly on eBay.
I've been using Paypal for years and about 500 transactions. I've never had a problem, its like a bank account. Once the money has been transferred into your Paypal account you are safe. Just make sure he pays you before you ship it. If you are shipping a computer make sure you package it very well.