From the legal side, ditto what Jack19 said. Basically, CYA. Make sure that you know the legal requirements for your location and do everything you have to, if not more. I live in Iowa, where you need an annual permit to purchase, no limit on purchases with that permit. Private sales do not require the permit, but I would not sell to anyone who did not have one. If selling, I would get a copy of the buyer's permit and driver's license. If buying, I would get a copy of the seller's Driver's license and a signed bill of sale.
From the practical side, you have four options: try to sell to an individual locally, sell to a dealer, trade with a dealer, try to sell online. A local private buyer may be the hardest to find, but will probably net you the best price, and will have the least hassle once the buyer is found. If you sell to a dealer, it will be quick and easy, but this will probably get you the lowest price. A dealer is more likely to give you a break if you trade toward something he is selling. But do your homework first. A gun show is a good place to get a number of bids in a short time. These offers can vary as much as $100 on a $500 gun. I have never dealt online. It adds many complications, including payment, shipping, and an FFL transfer. It's also a matter of personal preference: I would never buy a gun from an unknown private party without test firing it first.