This is not true. I've encountered Google Shopping offering to checkout right on Google Shopping, where underneath they wrap the third-party transaction for you

This is not true. I've encountered Google Shopping offering to checkout right on Google Shopping, where underneath they wrap the third-party transaction for you. They had some questionable sellers on there, were selling used goods, and they were even promoting it with discounts. It was probably a minor feature being trialed but the point is the line is not so clear.