One option is to use a check as "good faith".
A check is written by one party (normally the one who started the deal). Send the check. Upon receipt, the firearm is sent to an FFL. After receipt of firearm. the other firearm is sent with the check back to originator.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)