Here is the sentence again:

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.

Clarence bought four gallons of exterior latex for only ten dollars is a main clause. It can stand alone as a complete sentence. Then is a transitional word that is introducing the second complete sentence. Because these two parts run together with no punctuation, they are making a fused sentence, a major error in sentence structure.

A period after dollars and a capital T for then would fix the problem.

Go to the next sentence.

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