Use this form to change or amend terms of the Purchase and Sale agreement after it has been signed. A purchase and sale agreement is a contract between a buyer and seller for transacting a property in exchange for a specific price.This file contains detailed instructions for completing the NWMLS Form No. 21, a Residential Real Estate Purchase and Sale Agreement. A Massachusetts purchase agreement is the standard legal form for selling or purchasing a home. It documents the transaction's stipulations. Steps for Filling Out a purchase and sale agreement MA PDF Form. It is a step-by-step process to complete the purchase and sales form from MA. The completed form will relay the unique terms and conditions of the sale, including the purchase price and the manner of payment. Parties: the "SELLER," agrees to sell and. , the "BUYER," agrees to buy, the premises described in paragraph 2 on the terms set forth below.