I've used Gunbroker numerous times for buying and selling firearms and accessories. I WILL use them again when the time comes. They are legal, safe, clean, reliable, and do not put up with nonsense from sellers or buyers. I've found some incredible deals on there, and also gotten twice and more than I had expected to get when selling. My advice, when selling, check out completed sales on similar arms to get an idea of the current selling price before you list. That way, you'll not be too surprised. It also helps to put a reasonable starting bid on it. With more unusual items, I've put a starting bid at my minimum acceptable price,with a Buy it Now quite a bit higher... crazy high, in one case, and had it sold quicky with Buy it Now. Whoopie!!
As to the legal end of it.... YOU are a private party, thus do not need to have an FFL to sell. HOWEVER, when selling interstate, OR other than face to face, you MUST ship the arm to an FFL in the SAME STATE as your guyer. HE will decide whom to use for this "transfer" service.. unconstitutional, yes, but until it is changed it is not worth the risks involved. It is a good idea to get a copy (faxed, mailed along with payment, PDF attached to an email) of the FFL to which you are sending it, in case some crazy guy thinks you are selling interstate against the law. That way YOU have proof it went to an FFL. Not required, but a cover your six sort of deal.
For sales to California, it gets dicey. For ANYONE to ship any firearm into that state, it MUST go to an FFL, and the SHIPPER Must be on their Department of Justice list of "accepted and approved" sellers. No joke. SO, unless you are on the California DoJ list (you are almost certainly not), or you ship it through a local FFL who is (it will cost, but may be cheaper in the long run, later details), don't sell to California. Other states are fine... there it is up to the buyer to make arrangements on HIS end. He will know..... and make the needed arrangements.
Word of warning... you can NOT ship handguns via USPS. Illegal. An FFL can, though. In that case, Priority Flat Rate will get it anywhere for twelve bucks, Private parties can ONLY ship handguns interstate via UPS or FedEx... and THEIR OWN STUPID RULES mandate Overnight class... typically, eighty bucks or so. Thus, when buying, most astute buyers will not buy private party BECAUSE of this insane cost. However, if you have a local FFL who is willing to charge, say, twenty bucks to use HIS license outbound (it MUST be recorded in and out on his FFL sales book), add the twelve for Flat Rate, and you're out maybe $35 instead of eighty. Most pawnshops these days take the more valuable arms they end up with and sell them on Gunbroker anyway, maybe one of them will cooperate.
Long guns can be shipped USPS, however. Go figure. Sounds like a deal brokered by the Brady Brats. WHY UPS and FedEx insist on overnight, I do not know. Stupid....
Gunbroker can be used to great advantage, putting your wares before a nationwide audience. Just make certain your payment is secure in hand (as in, check cleared and IN your account) before you ship. Get the copy of the receiver's FFL, and ship ONLY to that address. Now yhou're covered. And beware of insane high costs to ship handguns. Or make arrangements with a local FFL (who MUST have receiver's FFL in hand before shipping interstate). Take the lowest price one and try it...
one more thing... grade honestly (I've always been surprised at how much BETTER my purchases have been than what I was expecting based on grading by seller), and take clear, sharp photos showing every side of the arm. Only as many as are needed.... good contrast, well lit.... most times it is the photo that sells the gun. And I've bought stuff with miserable photos for a song, because most others could not tell what was there.... and been shocked when it was far better than I had thought it would be. Or, in a different finish that what the photo appeared to show, worth twice what it appeared to be. (got lucky that time... oh boy.. a rare Brownign High Power in hard nickel chrome, when I thought it was only the matte nickel finish. And in as new condition, forty years old. Nice....