Like most if not all things, this carries a degree of risk.
Ask yourself and the potential buyer 2 vital questions:
1. How can payment be remitted?
2. How can the domain name's ownership be transferred?
While using paypal is a relatively simple solution, problems can occur if the
buyer turns out to be a scammer. So your minimal objective is to get as
much money out of the deal as possible.
This is where it makes sense to use some kind of "escrow" service. They'll
hold ownership of the domain name pending complete payment to you, or
even hold payment in the event a dispute is reported.
Two services I can suggest you have a look at are Escrow.com and also
Moniker.com. Between the 2, the latter might be more advantageous since
you're also dealing with a domain registrar, which Escrow isn't.
Do your independent research, as always. Good luck and good hunting.