Unless both you and the other party both have C&R licenses, any handgun you (or the other party) receives from another state must be transferred to you through an FFL in your state of residence. While technically the seller doesn't need to use an FFL on his end to ship the firearm, many FFLs decline to accept guns that don't come from another FFL on the originating end. If you're talking "trade" as in "I'll send you this if you send me that," then an FFL will be required on the other end for the other person to receive your handgun, so it's probably easiest to have both guns shipped and received by the respective FFLs.
