Hei Daniel, the good thing of Ethereum for this types of problems is the posibility of make smart contracts. Smart contracts provide you one option for make the peer to peer contracts more easy and withour any cost of third persons or entitys.
Then for your case i try explain you how work a little case, you have a car rental company this can make contracts of cars with for example id, model, color, state, cost for day and list of contrats of thsi car. You can put it in a smart contract code with solidity and you can make "N" diferents contracts (One for car, that you own).
So now, when you haver cars contracts you can do other smart contract with the name "contractOfCar" this is created by your other car contract and contain the initial rent date and the final rent day, and the state of the contract (Avaible or not).
In resum, you have this 2 smart conrtacts:
Model: model of the car
Price: price for day
Color: color of the car
Id: Id like registration number
State: For check if it car is avaible or not
ListofContracts: Array list with all contractCar of this car. (contracts directions on the blockchain)
IniDay: First date of the rent.
FinalDay: Final date of the rent.
People can make a contract of car with a web-app interface and you can store and watch it in the blockchain. With this you can have a contracts and the people can pay for rent your car with the web without third entity. (this is a very easy case, can be more complex like automatitzation of the payments, better smart contract structure, ...)