I've tested Basic cart and found it a very good solution for an online store without online payments. But I've only done tests, I have no experience with a real working site.
Most Promising looks Ubercart:
I congratulate the community for the work on Ubercart project! This is exactly the project that corresponds to the philosophy of Backdrop.
But Ubercart I have also not yet used in a working Backdrop site.
My assessment is due to the experience I have with real sites with Ubercart and Drupal Commerce with Drupal 7.
Ubercart is relatively easy, but Drupal commerce is promising, but very complex to configure and poorly documented. With countless unworkable, undocumented and abandoned modules.
But I have to mark two more projects in development, I do not know how far they will go, but I observe them periodically: