The eBay store builder extesnion was discontinued before Dreamweaver CS4 was released, it was not updated to compatible with that platform.
You will not be able to use the eBay store builder extesnion in Dreamweaver CS4.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.