Scene summary from the 1903 Edison Catalogue:
Showing a dock scene with bar room and ware houses on one side, and a large number of steam boats moored to the wharf. Scene opens with St. Clare's slaves singing, dancing, shooting crap, and otherwise enjoying themselves. The auctioneer appears, breaking up their sky-larking, and proceeds to open the sale. The first slave to be placed on the block is Adolph, the valet of the late St. Clare. Simon Legree, who is one of the bidders, steps up to Adolph, examines him, and spits upon his shoes because they are blackened. The bidding is very lively, and he is finally knocked down to Marks, who offers seventy-five. The auctioneer informs Marks that the terms are cash. Marks hands him a dollar bill and asks him for twenty-five cents change. The auctioneer is astonished, asking him if he expects to buy a slave for seventy-five cents. Giving him a cut with his whip, Marks makes a hasty exit. . . .

The scene of dancing and gambling slaves with which the auction sequence begins is not in Stowe's novel, but it began appearing in dramatizations of Uncle Tom's Cabin as early as 1852. To trace some of that history you can use the site's SEARCH MODE: enter "Conway" and "Barnum."