Help understanding database design logic for a secondhand bookstore project in MySQL
You don't sell to users, you sell to parties (companies or individuals). Some might play the role of users.
There are many parties involved in a sales/purchase order:


the vendor (you if you sell)
the buyer (you if you buy)
a party selling something on consignment
sales staff taking commission 
the party that placed the order
the party that took the order (a clerk)
the party that is paying for the order
the parties to whom to ship the order items



Do yourself a favour and get a data model pattern book and don't reinvent the wheel :)

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