Market is defined in terms of total customers, customers that you can target, and your immediate customer that you are trying to obtain. You must have some reliable reference to prove that this market exist or it will be just an assumption.
A single page with Why your product is needed by the customer, the value you offer, competitor analysis, sign-up, or expression of interest form.
As soon as a potential customer is interested in what you are offering, they will search you on the Internet. Your presence on all the important platforms will give them the confidence that you are reliable.
The actual contract startup will use to do the transaction with the customer.
Ability to accept credit cards, deposit checks, issue invoices, etc.