Great! This shouldn't be so hard, then. (Though now I do kind of want an item exchange system, but I think I can just squint over the alchemy mod for hints on that.)
My only foreseeable issue with your design above is in the displaying of items, for the user's sake - how will they know what the item does before they buy it? If you can, get me some more detail on what you want there. (Though if each shop has a specific type of item, there is much less confusion overall and descriptions may be irrelevant.)
Let's get done building how we want to display the page in plain HTML first, then I can work on converting it. ^^ As a start, this covers what you have in mind so far, -ish?
http://jsbin.com/jabufegula/1/edit?html,css,output (You can turn off tabs for the html/css/output displays at the top, as well as drag the widths of each of these windows. By default, the output window may be narrow, so drag it wider to get the full feel of it.)