See all orders from subsite in mainsite

I am building a website like the foodordering http://www.just-eat.co.uk or other examples.

Now every restaurant is going to have his/her own subsite with products. But I need to see all the orders and receive all the money.

At the end of the week I will pay out all restaurant owners minus my commission. But I have to see all cash orders to, so I can recalculate what they should get. Will be offering only Ideal.

If there is custom coding involved. Where to start this? And what to look out for?