Who are the people whose names are etched into the trees? Why were their names there?

The men whose names are scored on the bark of the trees were all wealthy men in the colony. They were men who'd gotten wealthy by dishonorable means or who'd acted dishonorably. In reading the text, we can infer that Old Scratch had something to do with their accumulation of wealth.

Tom looked in the direction that the stranger pointed, and beheld one of the great trees, fair and flourishing without, but rotten at the core, and saw that it had been nearly hewn through, so that the first high wind was likely to blow it down. On the bark of the tree was scored the name of Deacon Peabody, an eminent man who had waxed wealthy by driving shrewd bargains with the Indians. He now looked around, and found most of the tall trees marked with the name of some great man of the colony, and all more or less scored by the axe. The one on which he had been seated, and which had evidently just been hewn down, bore the name of Crowninshield; and he recollected a mighty rich man of that name, who made a vulgar display of wealth, which it was whispered he had acquired by buccaneering.