import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinColumns;
import javax.persistence.ManyToOne;

@Entity
public class Item {
   @Id
   private long item_id;

   private double price;
   
   @ManyToOne
   @JoinColumns({
      @JoinColumn(name="seller_firstName", referencedColumnName="firstName", updatable=false),
      @JoinColumn(name="seller_lastName", referencedColumnName="lastName", updatable=false),
      @JoinColumn(name="seller_birthday", referencedColumnName="birthday", updatable=false)
   })
   private User seller;
}

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity
@Table(name="Users", uniqueConstraints = {@UniqueConstraint(columnNames = {"firstName", "lastName", "birthday"})})
public class User {
   @Id
   private long user_id;
   
   private String firstName;
   private String lastName;
   private Date birthday;
   
   private String username;
   
   @OneToMany(mappedBy="seller")
   private Set<Item> itemsForAuction = new HashSet<Item>();
}