First of all, i'm glad to be part of your forum members! Hi to everybody...

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* represents primary key

As far as i see, this design is good. I need your confirmation I followed 1NF, 2NF, 3NF, and one to many relationships guidelines.

-Seller Table {*SellerID, first name, last name, address, city, CountryID (foreign key), phone number, date hired}

-Customer Table {*CustomerID, first name, last name, address, city, CountryID (foreign key), SellerID (foreign key) }

-Car Table {*CarID, manufacturer, model, car year}

-Purchased Car Table {*CarID (foreign key), *CustomerID (foreign key), date purchased }

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These tables have multivalued fields...

Country Table {*CountryID, country name}

A customer can store several phone number (home, cell, work):

Phone Table {*PhoneID, CustomerID (foreign key), phone number}

Relational databases are never multi-valued. Attributes are always single-valued attributes. Perhaps you meant something different though and if so then maybe you could clarify what you mean.

I think your design is a reasonable first attempt. You might want to reconsider possible dependencies between the Address, City, Country attributes.

maybe you could clarify what you mean:

You are right. I meant country field will have a drop list that contains country names, so basically the field will end up having one value.