Yikes.... If somebody asks (or even worse, demands) F&F, that's usually a sure sign to run the other way, as you have zero recourse through PayPal. Heck, you may be buying something from the most honest person on the planet. What if they get hit by a bus on their way to the Post Office to drop off your package?
I ship hundreds of items through PayPal, and even when buyers ask me if I prefer F&F I tell them to go ahead and pay with goods and services for their own protection. I have no intention of scamming anybody, but anything can happen in the mean time. Sure, PayPal takes their extra 3%, but we all signed the same terms of service when we signed up.
Now, depending on how you paid via PayPal (I.E., a credit card or a bank account), you MIGHT be able to dispute the charges through your credit card company or your bank, but PayPal isn't going to do a thing to help you, unfortunately.