Hi William,
It is only accessible for partners, but I am sure Kineo will be able to advise on solutions they have used.
Some highlights of the discussion in the partner thread:
Paypal which comes with Moodle/Totara is fine but limited, so partners have explored third party plugins or even built their own
Coursemerchant is used by some partners
Magento was recommended in this blog post
A custom solution, "We have built a Moodle e-commerce solution by integrating the Drupal CMS and Moodle. The advance to this approach was that we were able to use the Drupal Ubercart e-commerce module which is sophisticated and mature in terms of its ability to manage purchase logic."
Andy