It would be relatively simple to set up what you're looking to do. There are a couple add-ons in the marketplace that will greatly help with this:

Domain Mapper ($95) -http://www.concrete5.org/marketplace/addons/domain_mapper/...

Multiple Domains ($60) -http://www.concrete5.org/marketplace/addons/multiple-domains/...

Basically, these addons allow you to point sub-domains or regular domains to any part of your site.

Let's say you have "yourmainwebshop.com" as your main site where your concrete ecommerce is set up. Then, you could add a page like "stores", and add each store as its own page, so you would now have something like: "yourmainwebshop.com/stores/separatestore". Then, you would point a domain (or sub-domain) to that.

Also, you could assign a different theme to each of the "/separatestore" pages, so it would be like having a unique website for each.

I hope this helps?

Out of those 2 add-ons you recommended, is there one that you prefer? Also, is there a way to do "everything under this page, use this theme" or do I have to individually change the theme for each page?

Although both addons do essentially the same thing, I have heard that the Multiple Domains addon is better.

For each section of your site where you apply a different theme, it will automatically be applied to all pages added beneath it (unless you used a different one for a specific page).

I read the description for Multiple Domains, and the make it sound pretty intimidating. Is it difficult to setup?