What information do we need to create an E-Commerce platform?

Obviously, we will need to contact a web agency which provides a package of services (web developer, web design and graphic design) to create the e-commerce platform website itself.

But before that, what do we need to do or what information do we need to gather?

I'm sure many of you who are here must have tech skill and some of you must have some experience in creating e-commerce platform.

So I'm asking for those who have experience in e-commerce creation, could you tell us on the different information that we need to know for creating a retail online shop?

My questions:

1) [For those who have knowledge in LAW]
For a retailer, usually we need a trade license but for an e-commerce platform, what are the license we need, if different?

2) [For those who have knowledge in LAW]
What are the goods that we can not sell on an e-commerce retail online shop? Or good which needs special permits?
(I'm guessing that we can sell everything that a supermarket has; ranging from foodstuff, drugs-vitamin, to hardware-Quincaillerie and electronics)

3) [For those who have experience in e-commerce creation]
Around how much would an e-commerce platform website cost? E.g how much would a web agency charge for a website like priceguru or shop.cleverdodo ?

4) What is included in the list of expenses (Aside the cost of creating the platform) by the web agency?

I think those information would be very helpful, not only for me, but also if we are to get more online shopping in Mauritius.

P.S: I'm currently exploring the possibility of doing an commerce online.