PHP eCommerce Software - Best and least expensive.

I am looking for the best ecommerce software written in PHP.
I need to be able to customize the software.
The software will be used to display automobiles.
Some features:
-back end admin page to maintain inventory of vehicles
-credit score verification - user enters info to get credit score
-easy navigation of inventory of vehicles, drill down menu from main category to sub-categories
-grid which allows listing of all vehicles and user can dynamically sort by clicking on a column
-slideshow to view vehicle
-360 degree capable viewing of vehicle
-able to attach documents to each inventory item, for example, pdf file of vehicle care sheet.
-MUST be SEO compliant, that is, list of inventory should have SEO html format
-interface to google desirable
-interface to eBay desirable