Web API model classes

I'm coding a webshop using ASP.NET Core Web API for the backend and Next.js for the frontend. I'm designing an Entity Relation Diagram (ER-diagram), but I'm not sure if I need a user class, since the customers of the webshop don't have to authenticate themselves in order to purchase stuff. I'm thinking of configuring some local storage in the frontend, but I was wondering what impact that would have on the ER-diagram.
Is this homework/learning or for a professional project? If this is a professional project, then how will get around needing a user entity? Can someone cancel an order/transaction after its submitted? How would you confirm a valid cancel request from the user of the site from an unrelated third party? What do you need an entity relationship diagram for? I've usually only seen that used to explain database relationships between entities, or as a communication tool for specific purposes.

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