Make darn sure you get pictures before you buy the firearm! Then negoiate an inspection period, ie three business days, a non-shooting inspection. Agree to terms of return, ie who pays the return shipping and FFL fees?
1 All interstate transfers of firearms need to shipped to a FFL holder!
1a Some receiving FFL holders will not accept a firearm from a non-ffl holder ie a seller, as that is their choice in doing business.
1b You need to find a local ffl holder to receive the firearm, look on Gunbroker or Shotgun News web sites for lists of FFL holders that will do transfers for a reasonable fee.
1c Send by mail or Fax a copy of receiving FFL to the seller or their FFL holder so they know where to ship the gun.
1d Make sure you get a tracking number so that you know when your three day inspection starts. Make darn sure you are there to look at the gun at the receiving dealer with in three business days. Not that he can send it back to the seller, at this point. But, you will have to take delivery of the gun, and then send it back to his receiving FFL if he is not a FFL holder.
2 Arranging payment for the firearm well here is where you have to take a leap of faith. Some will say use USPS money order, in theory mail fraud occurs if the seller absconds with money. You can stop payment on Money Orders. A cashier's check works too, some will take your personal check and hold for clearance for 10 business days.
3 Get a bill of sale laid out and emailed to you from the seller, with a copy enclosed with the gun. Spell out all the particulars! The sellers name, address, phone #. The shipping method and tracking # vs deliver confirmation #. The shipping FFL information if needed. What you are getting IE
1 Rem 700 VS 223 SNXXXXX with Leupold 2 pc PRW bases, PRW rings, along with orginal factory paper work.
2 RCBS Standard Die Set PN#XXXXX
3 200 pcs of Lapua brass in MTM case guards