You will both have to ship to an FFL legally anyways
First thing to do is check the Feedback section for positive feedback and also to confirm that they dont reside in the Hall Of Shame. Dont just check screen name check real name.
Second thing to do is make sure you have verified contact info for the other party
Third thing to do is invest the buck or two and call them. Make sure everything feels right.
Fourth, agree in writing to terms of your deal and include inspection period and what happens if one party is not happy with the deal
Fifth, ship the gun
Sixth, come back and post positive feedback after your deal goes silky smooth