Seriously, good pics will get you the best price, especially on online auction or forums where the buyer maybe a long way away. There are enough poorly photographed guns for sale that most view them as a gamble, cause you are not sure what you are getting. Therefore they won't be willing to shell out the cash for an unknown factor.
Get a cheap tripod or borrow one. Set the camera to a timer so you aren't touching it when it goes off. Make sure it is in focus. PM me if you have some questions about specifics, or maybe any of the others on here who take good pics.
In a description, if there is a problem with the gun, or if it has been reblued, let the buyer know. It'll save in hassle and bad feelings.
Caliber, barrel length, date of manufacture if possible, accessories, are all pertinent information. Box and papers, if they are included.
Location and face to face only, or if you are willing to ship, a price on that.
That should get ya started!