Well I'm building a site for a client on the Expression Engine CMS and just wondering if anyone had any experience building a shopping cart on it, either custom or via a plug in.
Expression Engine comes with the Simple Commerce Module that should satisfy the most common needs for a shopping cart. It utilizes the Paypal IPN service.
Also check out Mark Bowen's Shopping Cart plugin, which works with EE's Simple Commerce Module and provides extra functionality.
I'm just starting an EE/Ecommerce project using the FoxEE module (http://www.hccdevelopment.com/Web-Based-Software/Product/FoxEE/) which ties in with the FoxyCart. It looks to be a very comprehensive option for ecommerce. Much better then the Simple Commerce Module.
I used Foxee with Expression Engine and Foxycart on this site: http://prints995.com and it worked pretty well. The latest version of FoxEE is pretty good.
There is a new one recently released, named Cartthrob - Which allow you to build your store with the same kind of flexibility as the rest of EE. It comes delivered with no assumptions about your setup and wishes, and consist of a set of EE tags.