Over the years I have bought a couple of rifles from them - one a Lee Speed. Their buyer's premium is high, but unfortunately, in line with many other auction houses. Their "sealed bid" is essentially the same as other auctions. You place your maximum and on auction day hope no one outbids you.
The collecting of high value American firearms has more in common with coin collecting than finding good guns. Value is based upon rarity of the particular model, desirability from a collector's point of view, and accurate percent of original condition. It is why their is something of a cottage industry in faking condition. Also, because there are relatively more of them, the collecting base is much larger for let's say a Model 1873 Winchester than a WR Droplock. That, in turn, drives competition and prices.
European guns, which were often refreshed by the gunmaker, tend to be priced according to current condition and user rather than collector desirability. They tend to have collector value when they are associated with a particular desirable provenance.
For instance, a nice WR .577 double with full coverage rose and scroll engraving in 85% condition will sell at auction for between $35 and 50 thousand. Ernest Hemingway's sold through Julia in 2011 for $340,000.