WPMU with Gridmarket Theme with MarketPress

I have established a "boutique" community at http://www.TheBoutiki.com. I was looking to keep it as simple as possible, especially since I really have no idea what I'm doing when it comes to css, themes, and such. Originally I was hoping to start an Etsy type site where people had their own pages and sub-domains where they could add their own products without having to deal with the Wordpress back-end. For now I am posting all of the products and I am the only one able to log in since I had given up on my previous plan for many complicated reasons. I figure that I'll give it another go, now, since my application with Paypal for chained payments has been accepted.

I have successfully completed the transition from a single Wordpress site to a multisite installation. My theme is just how I would like it, though I do require a simple way for my vendors to register, create their own site, and maintain their postings from the front-end, similar to Etsy.

Also, when I do create a new site, such as the one I have created at http://www.example.theboutiki.com, I found myself having to re-create the theme I customized on my original site. Is there any way to simply make all of those customizations come about automatically when creating a new site? I have created a dummy administrator for that site.

I'm hoping to make all of these changes smoothly as I already have a steady stream of regular visitors and would prefer not to make too many cosmetic changes.

Thanks,

Jake B