Hey Xport,
I understand what you are saying, as I myself will be tackling something very very similar but on a much larger scale (boardgames.ca, tradingcardgames.ca, miniaturegames.ca, roleplayinggames.ca, pokergames.ca)
Thus, i'll be looking into acheiving something similar.
However, I question why you would want to have it on the exact same domain, with a /relevantmarket ? This doesn't really seperate it out. It would really be better for your customers, if they were different domain names.
But, how you could do it is make your very own HTML pages, and manually put the product information on them, and the unique design for Tile/Marble, and all of that. I'm sure

can do a more detailed description of this process for you (or they already have somewhere on the forums) I know you can use DRAW's to call the Shopping cart, and the Buy button, and all of that. So you could really build your own couple stores manually, so they can look and feel different.
I do wish however, that the databases of multiple stores could somehow interact. This is because all of these online stores will be run from one Brick and Mortar store using their single Point of Sale system, that we've had to jury rig to make work with

(and we are still working on that). So then having to do that again for new stores will be a pain, but hey... like Ryan said, if it's not making more than $60 a month (the store plus dataport fee) then is it really worth it to be pursuing it anyway?