SQL server 2012 how to apply a check constraint on multiple columns
You can create a table-level check constraint with multiple columns specified in the expression in order to avoid separate check constraints. 

This example will not allow an empty string, all spaces, or NULL in these columns:

CREATE TABLE dbo.Buyer(
      BuyerId int NOT NULL
        CONSTRAINT PK_Buyer PRIMARY KEY
    , BuyerName varchar(50) NOT NULL
    , BuyerSurname varchar(50) NOT NULL
    , BuyerEmail varchar(50) NULL
    , BuyerAddress varchar(50) NOT NULL
    , CONSTRAINT CK_Buyer_BuyerName_BuyerSurname_BuyerEmail_NotEmpty CHECK
        ('' NOT IN (BuyerName,BuyerSurname,BuyerEmail) )
    );