Here is the item 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 conjunctive adverb 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.