Read about the legal terms and disclosures you must add to an Illinois real estate purchase agreement. Learn what required information you must include.Purchase and Sale Agreement. A purchase and sale agreement is a contract between a buyer and seller for transacting a property in exchange for a specific price. Get a comprehensive understanding of Illinois real estate purchase and sale agreements with our indepth overview and free template. 1. Enter the names of the Purchaser and Seller. Before filling in the details of your real estate purchase agreement, identify the state where you will execute the contract. An Illinois residential purchase agreement is a legally binding contract between the buyer and seller of a residential property.