I would suggest looking at the supporters of this site. I would write them a letter, listing the guns, and asking of their interest in handling the sale.
I would also ask their fee and negotiate from there.
The fact that this is a collection, much can be gained by the agent or house in advertising that. Generates great interest.
Hope that this is helpful.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.