AS AUCTIONEER BILL STAHL brought down the gavel for the 432nd time on Saturday, January 29, virtually everyone in the Manhattan salesroom of Sotheby’s knew that they had witnessed history in the making. Sotheby’s sale of paintings, folk art, and other objects from the collection of Bertram K. and Nina Fletcher Little was a jaw-dropper, bringing in nearly $7.4 million — more than double the high estimate of $3.6 million and more than double the previous record for a collection of Americana.