Make sure you enable Ecommerce in view settings and add the extra Ecommerce code to your site or application.
A simple way to check which marketing channels is driving sales to which products is by selection your products and then add Source/ Medium as a secondary dimension
Sales performance is good to identify trends (which weekdays and hours you should target with marketing).
Transactions let you dig into a specific order and see which items get ordered together a lot. This info can be used to cross-sell products and extremely effective advertising technique to create bundles etc.
You can read more about Google Analytics for Ecommerce here: