Uncomplicated. Simple. Easy. Yet powerful.
Pluggable and extendable. Ships with several free addons to start with.
Fast for the site developer to setup, easy for the site owner to use.
PayPal Express Integration. Supports the PayPal Sandbox too.
Stripe Checkout Integration. Easily supports testing.
Hosted on this site is a pretty detailed list of docs and tutorials ready to take you from zero to your first sale.