Take a look at the sentence again:
Clarence couldn't believe that the hardware store was selling garden rakes
for only $1.99 each when he brought home
twenty of them, his wife Marie just shook her head and squeezed them into
a garage stuffed full of her husband's other "good buys."
The problem occurs between each and when. Clarence couldn't believe … begins a complete sentence. When he brought home twenty of them, his wife Marie just shook her head … starts the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after each and then capitalize the W for when.