Talk to local auctioneers about their experience with gun auctions. Online timed auctions and in person auctions people are paying good money, especially older and some specific caliber guns. You could have the buyer pay a buyer premium to cover cost or auctioneer takes percentage, depends on the company. 400 guns could be split into 2 auctions, but an experienced auction company could handle 400 in a timed online auction. Where I live there at least 3 auction companies that do really good organized online auctions, I follow them, online auction seem to be the way to go.