Validating Uniqueness With Data Annotations In Mvc

I have various questions about validation using data annotations. I am using the following setup

asp.net mvc 2
entity framework 4
data annotations

Basically, I'm trying to get unique validation working and i'm a little confused. My models are as follows:

public class Buyer
{
public int Id { get; set; }
[Required(ErrorMessage = "The email is required")]
public string Email { get; set; }
[Required(ErrorMessage= "The name is required")]
public string Name { get; set; }
}
public class Seller
{
public int Id { get; set; }
[Required(ErrorMessage = "The email is required")]
public string Email { get; set; }
[Required(ErrorMessage= "The name is required")]
public string Name { get; set; }
}...............................................