Never be afraid to ask for references if a person doesn't have an active post count or selling history.
Ask for a ph. # and contact info and call to feel the person out by phone.
Use the contact info and try to see if everything matches up. You can use the internet search engines to look up a ph. # and address or take a ph. # and try using reverse look-up (these won't help if the # is un-listed of course).
In instances where an FFL is needed call the FFL and see if he knows the seller of the firearm or has been contacted by the seller to handle the transaction. You have some leverage here so use it wisely.