An eStore is any nice addition to a topic or niche based site. Most people set up eStores and never see a single sale. This is because no one knows their store exists and who buys things off of random internet stores?
Going a step further
Now, the important part here is that you build a site to house your eStore. For instance, my actual eStore is located here but I have built a site that utilizes iframes so my Amazon eStore is actually an entire site that hosts individual sections for various sites of different niches.