One site to control all. Only allow new users to add/edit products

Hi, firstly i'm on a local install so cant link to my site just yet.

I realize this has been somewhat asked and answered in many threads, but after reading through hundreds I decided I should just start my own.

I want to create a site/store that allows the public to list and sell items within it. So due to the % payment model I have been forced to go with a multi-site set up. This isnt ideal, but it seems like its the way to accomplish what I need so hey ho.

Really I only want one store front. Ensuring each child site (im using folders) follows my main sites theme should suffice here. I may run into some trouble with global listings etc, but ill get to that later if necessary.

I need a new user to be able to list their product for sale, they would ideally hit a list item/sell item button, fill out a front end form (most of which will be created by custom fields ACF). This item would then become 'pending for sale' until we ok it to go live.

As im seemingly forced to go the multi-site route here, I need too be able to lock out new users/sellers from doing anything but listing products and further down the road, actually selling the product. So no plugin, theme, style editing etc etc, just list product (filling out our form) and then being alerted to offers or sales on said product for them to be able to deal with.

If I didnt have to use multi-site for payment partitions, then it would all run under one shop. As it is, I can see a seperate shop for each seller being useful to potentially allow the user to view, control and edit their listings as needs be.

But everything else, bar the listing and controlling of products should follow my main site. Headers, nav side bars etc etc etc, all controlled by us.

They list a product, we ok it, it filters in to the main store listings. Bish bash bosh.

Im currently using SimpleMarket theme, but can change to the framemarket if required.

I hope you can point me in a solid direction to undertake this task. Cause at the moment im flailing around in the dark, trying different things.