Take another look at the sentence:
At a garage sale, Clarence bought four gallons of exterior latex for only ten dollars then, when his neighbors saw the nuclear pink, they collected 120 dollars to buy four gallons of primer and four gallons of forest green paint.
They collected 120 dollars begins a main clause. To buy four gallons of primer is an infinitive phrase. When an infinitive phrase completes the idea of the main clause in front, you need no punctuation to join the two.