The author concludes that Wilton was guilty of fraud. This is based on the following:

If one sells an item that they know is defective, but tells the buyer that the item is not defective, that constitutes fraud.

Wilton sold a bicycle to Harris, without knowing whether it was defective.

Wilton told Harris the bicycle was not defective, but it turned out to be defective.

The author assumes that Wilton knew the bicycle was defective. But all we know is that Wilton didn’t know about the bicycle’s condition. He might not have known that it was defetive.