How did Tom get Ben to paint the fence in "The Adventures of Tom Sawyer"?

In this part of the story Aunt Polly has attributed Tom the task of whitewashing her fence. Of course Tom would rather be out fishing, swimming or just hanging out with his pals, but he has been 'conscripted.' One boy has already come by (on his way to the swimming hole), so when Ben shows up, Tom has already worked on a plan. He first ignores Ben's teasing him, then looks up finally to say he hadn't even noticed him. When Ben asks if it's not "work" that he's doing, Tom hesitates and then states that maybe for some it is but whitewashing suits him just fine. Then when Ben wants to have a go at it just to see what he's missing, Tom refuses but finally "mellows." He gets so convincing about all the fun he is having that before the afternoon is over, he has bartered turns at whitewashing with all the boys that come along in exchange for "boy stuff" that any kid his age would covet:

Tom gave up the brush with reluctance in his face, but alacrity in his heart. And while the late steamer Big Missouri worked and sweated in the sun, the retired artist sat on a barrel in the shade close by, dangled his legs munched his apple, and planned the slaughter of more innocents. There was no lack of material; boys happened along every little while; they came to jeer, but remained to whitewash. By the time Ben was fagged out, Tom had traded the next chance to Billy fisher for a kite in good repair; and when he played out, Johnny Miller bought in for a dead rat and a string to sing it with - and so on, hour after hour. And when the middle of the afternoon came, from being a poor poverty-stricken boy in the morning, Tom was literally rolling wealth. He had, besides the things before mentioned, twelve marbles, part of a jew’s-harp, a piece of blue bottle-glass to look through, a spoon cannon, a key that wouldn’t unlock anything, a fragment of chalk, a glass stopper of a decanter, a tin soldier, a couple of tadpoles, a kitten with only one eye, a brass door-knob, a dog-collar-but no dog - the handle of a knife, four pieces of orange-peel, and a dilapidated window-sash.
He had had a nice, good, idle time all the while - plenty of company - and the fence had three coats of whitewash on it! If he hadn’t run out of whitewash, he would have bankrupted every boy in the village.

Tom learns an important lesson in negotiation through this experience -  a pinch of reverse psychology mixed with the sway of the group over the individual can go a long way. Aunt Polly's whitewashing job as punishment turns out to be the most "profitable" day of Tom's whole summer.