What are basic business rules for the database design and how to use those rules to identify the referential integrity constraints, default values, indexes and data formats.

I have a data dictionary created for online auction website, but need to update them with new constraints, default values and data formats. Please help. This is the basic data dictionary I created.

\begin{tabular}{|l|l|} \hline BuyerTable & Data Type \\ \hline Buyer_ID & INT(10) \\ \hline First_Name & VARCHAR(30) \\ Last_Name & VARCHAR(30) \\ Email & VARCHAR(30) \\ Street & VARCHAR(30) \\ City & VARCHAR(30) \\ Zip & INT(10) \\ Country & VARCHAR(30) \\ Phone & BIGINT(10) \\ \hline Gender & VARCHAR(10) \\ Password & VARCHAR(10) \\ \hline \end{tabular} \begin{tabular}{|l|l|} \hline Product Table & Data Type \\ \hline Item_ID & INT(10) \\ \hline Description & VARCHAR(30) \\ \hline Item_Name & VARCHAR(30) \\ \hline Price & DOUBLE(10) \\ \hline Seller_ID & INT(10) \\ \hline Category_ID & INT(10) \\ \hline & \\ \hline \end{tabular} \begin{tabular}{|l|l|} \hline Seller Table & Data Type \\ \hline Seller_ID & INT(10) \\ \hline First_Name & VARCHAR(30) \\ Last_Name & VARCHAR(30) \\ Email & VARCHAR(30) \\ Street & VARCHAR(30) \\ City & VARCHAR(30) \\ Zip & INT(10) \\ Country & VARCHAR(30) \\ Phone & BIGINT(20) \\ Gender & VARCHAR(10) \\ Password & VARCHAR(10) \\ Payment_Method & VARCHAR(20) \\ \hline Bank_Account_Number & BIGINT(16) \\ Bank_Name & VARCHAR(30) \\ BSB_Number & INT(6) \\ \hline \end{tabular} \begin{tabular}{|l|l|} \hline Payment Table & Data Type \\ \hline Payment_ID & INT(10) \\ \hline ITEM_ID & INT(10) \\ Buyer_ID & INT(10) \\ Seller_ID & INT(10) \\ Payment_Method & VARCHAR(20) \\ \cline { 2 - 2 } Incident_Costs & DOUBLE(10) \\ Payment_Date & DATETIME \\ \hline \end{tabular}