Comparing products in multi vendor marketplace its really helpful to customers to choice right/cheap product in your marketplace. If you are looking to introduce add-to-compare feature here I explained how to do that.
YITH WooCommerce Compare plugin is an common best supportive plugin for WooCommerce. This will allow users to compare products of your shop. All products are saved in one complete table where the user can see the difference between the products. This plugin will compare attributes of multiple products. Also there is an widget avaialble to list the products users have added, where you can manage them. Also you can simply customize the compare table with your style, by editing the specific template.