They "register" on the site but don't enter billing information at registration. They then can't get products in to cart & there's no explanation that they need to enter full billing information to access secure payment gateways / shopping cart.
They haven't registered on site & can't add to cart
Under both scenarios people are now used to being able to register / checkout as a guest as preferred at the point of purchase as per most other online platforms.
In with this you still need to address the long standing issue of being able to register using social media credentials in line with the rest of the e-commerce world