Creating a Curated Etsy-like Site with Third-Party Themes

There are a few forum discussions covering how to create "Etsy-like" sites using the MarketPress plugin with the WPMU Framework theme.

I'm looking to build a similar site allowing users to create sites using third-party themes; i.e. non-WPMU themes. Users are expected to build their sites for either (1) product selling with a checkout or (2) information only. If it's relevant to this stage of development, please note that the network admin will collect a percentage of sales from the former case.

I've installed third-party themes for the network homepage and enabled them for the subdomain's pages. I've also setup a couple subsites for testing.

Please advise on steps to accomplish our targeted uses and if any further information or case uses is needed. If this query is too broad, feel free to narrow the scope and I'll open separate tickets.

Many thanks.