Gunbroker.com is a good place for national trade, but I believe they take a percentage of the price you get. In my state (UT) we have a website for statewide firearms classifieds. And with those you don't normally have to pay anything. I'd check local gun stores and see if they know of an online classifieds like that. I personally don't worry about selling guns through an ffl, just get the guys name and drivers license number written down for your records, along with the serial number of the gun. Unless you really want to make sure it goes to someone that can have it, then most firearms stores can do a transfer for a fee (usually $25).