A legal contract that obligates a buyer to buy and a seller to sell a product or service. SPAs are found in all types of businesses but are most often associated with real estate deals as a way of finalizing the interests of both parties before closing the deal. |||Sales and purchase agreements are also found in the upper supply chains of many large, publicly-traded companies. They are set up to help both the suppliers and the purchasers forecast demand and costs, and become increasingly important as the size of the deals increases.