378606 Multisites linked together.

I need the structure setup for a chain of sites on the same server. A way to link 9 sites in total on my server (I will build the sites myself using Drupal, unless this in not a good idea), They need to be all linked together (so I presume using the same mysql database unless this is not a good idea) and one of them will be a "multi-vendor" estore where users will have their own store page/area inside it (my favourite is Interspire Virtual Mall but I am not sure if it is possible to integrate the Interspire system into a Drupal CMS or if Interspire has a registering and payment for users to have their own store that is automated. In short, a way to interlink 8 Drupal sites to a Multi-vendor shopping Mall that all use the same SSO and an "open a shop inside the estore" function that is fully automated. The successful person to help me with my project will be able to tell me why their method to achieve the above will be the most adaptable for any future changes and why it will run the most stable. Please ff free toask me any questions if you need to know more about this project.