DataHawk's powerful Best Sellers Rank browser allows you to discover the top 100 selling products in up to 3 Amazon Browse nodes or subcategories. Here is a 4 minutes video overview for you: 

For instance:

You're also able to filter the result on an ascending or descending order on products price for instance, and able to Track a product you're interested in monitoring directly from the list. By tracking the product, you'll be able to unveil some of its metrics and monitor its performance on a daily basis. A form will show up to allow you to choose in which Project you want to track the product.