Contract for Sale of Real Estate. This form is an agreement between the purchaser(s) and seller(s), under a real estate purchase and sale agreement to purchase said purchaser's rights under said agreement.
While it's not a requirement, having a lawyer look over the agreement can be a wise move. They can help make sure everything is above board and that you're not missing any important details.
If either party pulls the rug out from under the deal after signing, it could lead to legal issues. Generally, the terms of the agreement will outline what happens in such cases.