There's no way to rate or comment directly in the marketplace product, but often creators make a post in the Roll20 Marketplace & Creative Content forums, so you should be able to search for comments there (or comment yourself) or create a new topic if no such post exists. Recently Publisher Pages have been added for marketplace artists, so hopefully other new improvements will be coming soon as well. I believe besides the rating system, a wishlist has also been widely requested. Cheers!