I've been buying and selling on GunBroker for about 12 years or so.
First, before listing your item do your research. Look at every comparable offered, and also check closed auctions to see what they are actually selling for (as opposed to the wishful thinking of some sellers).
Take good photos. Put those up with your ad to clearly show condition of your piece, including any defects, scratches, dents, etc. Keep a zip file of photos to send out on request to interested parties. Be critical of your own item, describing it openly and honestly. Most knowledgeable buyers will appreciate this and bid on yours before they will bid on others.
Set a realistic starting bid, the minimum you will consider accepting for your item. I also like to put in a "buy now" price toward the upper end of market value so that the guy who really wants what I have can buy it quick and clean.
Antiques and collectibles: Watch out for parts thieves. Some people will purchase your auction item to take what they need to restore another piece, reassemble yours with their old broken or missing parts, and demand a refund. Make it very clear that all sales are final, ask all questions prior to bidding, no returns or refunds, etc.
Shipping: Federal law allows you to ship to a FFL holder anywhere in the US. Many FFL holders will only accept shipments from other FFL holders. Make sure you know in advance what to expect, because transfer fees can be considerable, especially if there are several hands in the cookie jar. Long guns can be shipped via common carrier (UPS, FedEx, DSL, etc) using ground delivery (least expensive option). Most common carriers require handguns to be shipped "next day air", which is quite expensive.
Be prepared to provide an itemized bill of sale, notarized if asked for, along with a photocopy of your drivers license (sensitive information redacted). Some dealers will accept this in lieu of shipping via your local FFL holder.
Your local FFL can provide some other options for handguns. FFL to FFL shipments are allowed to use Priority Mail service. So even if you are paying your FFL dealer $25 or $30 to transfer the piece, and $12 or $15 for Priority Mail service, that is still a lot less than $70 or $90 for next-day air service by FedEx or UPS.
Regardless of how I ship a firearm I always insist on "signature delivery confirmation". I don't want a package left on someone's front porch or by the back door overnight.
Over the years I have kept records of dozens of such transactions. Those records will clearly demonstrate that I have not only complied with the requirements of the law, I have repeatedly exceeded the minimum requirements. In the event any question is ever raised I believe that I can demonstrate that I have gone out of my way to make sure every transaction was handled legally and responsibly.