I have been doing an evaluation of the OpenCart open source shopping cart software. It is implemented using PHP and MySQL. It is still early, but it appears to be a capable shopping cart. I have worked with Magento in the past and OpenCart compares favorably. OpenCart’s file and table layouts seem reasonable. I got lost when I was looking for items in Magento. The biggest thing that appears to be missing from OpenCart is an API for external interaction. With Magento, I was able to use an API for adding new inventory or retrieving new orders. I haven’t found an exposed API for OpenCart yet. However, the table layouts are straightforward and I was able to write an order export script using MySQL commands and accessing the database directly.
Like I said, it is still early in the evaluation process, but I would recommend adding it to your list of packages to investigate.