What is the ruling on taking pictures of products from e-commerce websites where the product is purchased, in order to use them for selling these products online?

There is no objection to taking a permissible image from a product sales website and using it to display the same product for sale. This is common, and sellers usually do not prohibit it, as their purpose in placing the image is to promote the sale of the merchandise. Customary permission in this case is like verbal permission.