Write down your main features as bullet points:

* User authentication * Users can post products to be sold * Making orders to _other_ users

Move on to the details of those features

* User authentication -> Social login OR email authentication -> Usual features like reset password/forgot -> Most other features will only be available when authenticated

I try to keep progress measured by features and use kanban boards to keep track of it. I don't go overboard with keeping everything in their lane and the checklists updated because I tend to not update things as much as I wanted to.