If you're buying the car, you will be expected to sign a document called the Retail Installment Sales Contract (RISC). In this post, we'll explore everything you need to know about car purchase contracts, cancellation agreements, and your rights as a buyer.Porsche Riverside is an authorized Porsche Lease Return Center and we are here to explain your options. When buying from a third party, you're responsible for making sure all the paperwork is correct, bill of sale, title transfer, and so on. How to Sell a Car in.