Welcome to the forum, Tom. This is a good place to have lots of exposure to the vintage doublegun "community". You should put the advertisement in the Doubleguns For Sale sub-forum here, along with all of the details on the gun that you can garner. The more info you can provide the easier it will be for potential buyers to discern what you have. Chamber length, barrel length, bore diameter, choke constrictions, length of pull, drop at comb and drop at heel, butt treatment, whether fluid or pattern welded barrels, and minimum barrel wall thickness are among the things most knowledgeable potential buyers will want to know. Then, good close up pictures should be posted here. if you need some help with that some of us here will be glad to assist you.
SRH