Understand your disappoint and frustration. But, take it easy, even we change purchase decisions all the time - that is why there is always return and exchange department in stores.
The key point is what is the cost as we think making change decisions. For large sales like cars or houses, there is always a deposit or legal liability to force us to give up the change decision. But, this does not mean nobody ever ate the bitter bullets. That is just alright for both situations.
The idea is that as you reach an agreement you always think about the possibility of deal changes, then work out an agreement condition: deposit for large sales, reservation period for popular items, or even first-come-first-serve...
Think about the case from another different angel: since the buyer already made up mind to quit, which possibly means he/she did not want the car anymore for any reasons, would it be comfortable for us to force him/her to keep it?
Happy Easter!