Do a search for something comparable that just sold. The guys who get the most have great pictures, extensive details and reasonable shipping.
1) if you're selling a pistol, it will be difficult to offer reasonable shipping unless you have an FFL, or know one willing to ship for you. Only FFLs can ship by USPS. The other carriers impost their own pricey rules on shipping, and it's not competitive.
2) if you're selling a rifle/shotgun, and the buyer's FFL will only accept a shipment from another FFL (many impose this as their own rule), you will also need to get an FFL to ship it.
So if you can work out the shipping, and it's reasonable and competitive. And you can take great pictures, you're likely to get top dollar for your sale. Otherwise, I'd say you're better off selling face to face.