ok, i just did a third party and it was my second. ill explain this one: i would use animalpaintball.com they are very reliable, work on it fast, get the job done, and great people.
you call Mondo(animalpb gun tech) and tell him your name and his name and that you want to trade guns. then you tell the other guy to get his gun and what it comes with into a box, then he has to include a letter saying that its going to you, and what your gun is supposed to have on it. then you do the same thing, package it up with a letter of what your gun has onit, and what his should have. then you also EACH include a check/MO/credit card for $25 for the third party. along with that you have to pay for the shipping of the gun back to you. calling mondo he can figure out what it costs, so you will know how much to include. so your gun and everything in a box, with a letter, with a MO/check/or credit card. same thing for him. then you send yours to mondo and he sends his to mondo. when he gets them he makes sure both work, and sends them to the oposite person. if it doesnt work, you can call off the trade and get your gun back, or he could prob. fix it. it depends on what type of gun it is though. thats pretty much how it works. i would use animalpaintball.com they are some of the best paintball people out there.