How do I best structure a site for 100+ users?

Hi guys, I'm trying to find the best way to build a site where we have over 100 sellers. Each will want to create pages and posts within their own area, but they will not need access to anyone else's area. I want the whole structure available to search across all sellers. And I want the maximum SEO value as if it's all a single site. Is that possible with the tools we have in wpmu?