How to make an ordering system in an online store?

Question

PostgreSQL

I am making an engine for an online store. The task is that - to implement system of orders.
That is, there is a seller and a buyer. The buyer wants to buy goods, makes an order. This order comes as a message or notification to the seller, he sends the order and changes the order status to "on the way". In general, about the same as on Aliexpress. How can this be implemented?