Some of the larger auction houses really only want guns that will sell for $2k or more. They often "group" less expensive guns into a single selling lot to reach that price level.

You might want to consider using two different auctions; one for the "deluxe guns" over $2k, and a smaller local house for the run-of-the-mill lower priced items.

17% to 20% seems to be a standard selling commission.