Woocommerce. The most popular e-commerce platform on Earth. Seriously, whenever a customer asks for something relatively simple that I know can be implemented

Woocommerce. The most popular e-commerce platform on Earth. Seriously, whenever a customer asks for something relatively simple that I know can be implemented using WP, I usually do it. You already have user management and many other functions built in, and extending it is very easy. It comes very handy when all the customer wants is basically a CRUD connecting to some internal system.