You would need to create another column to mark it as a conversion. set the default value of the conversion column to 0, when you update the record from a lead to a sale, updater this column to a one as well.
You will only need to have a foreign key relation from the from userstbl to productstbl if the user can be related to different products than the client. If the user is related only to the same products that the client is related, there is no need to relate the user table to the products table.