Probably the safest way to sell AND get top dollar is to go through one of the big auction houses that do guns.
These are internationally known houses who have qualified appraisers on staff.
The big ones are companies like Butterfield & Butterfield, Christy's, and Sotheby's.
If you have a very high dollar firearm, they're who usually auction it.
There are a few that specialize in only guns. One is:
http://www.rockislandauction.com/history.aspx