You cannot buy a firearm online excepting if both parties hold a valid FFL and all the proper paperwork (and there is reams of it involved in an online gun transaction) is filled out in either triplicate or quadruplicate (can't remember which it is...)
I reckon if the two parties involved were reasonably close to one another and planned on doing the actual transaction face-to-face it would be a different story as the regulations regarding private party gun sales are considerably more lax than buying from a dealer (whether it be new, used, pawn shop, whatever).
Just an //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif