When you buy a car at the dealership, you'll need to sign the car purchase or sale agreement along with other documents. Find out what you'll sign and why.Watch this before you finance your next car (former dealer explains how to read a purchase contract) Customer: I purchased a vehicle in Florida on December 2023 and I agreed and signed all the terms and conditions on the purchase contract. Vehicle Payment Plan Agreement. A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. The final hurdle you face is signing the vehicle purchase agreement; a confusing and convoluted document that we explain in detail here.