I will disagree with with the majority of the opinons here. A contract is offer, concideration and acceptance. I will sell the domain (offer) for $$$ (concideration), you reply "yes" (acceptance). This IS a contract. A contract cna be orally, over a handshake, in email, on paper, through a lawyer, etc. As long as those 3 elements are covered, it is a contraqct. Too many domainers are dillusioned to what an actual contract is or isn't. It does not have to be signed on paper to be valid.
Now, the part that is tricky, is it an enforcable contract? and is it worth it to pursue in the court of law to make the other person adhere to the deal? That is up to each person. Usually, people just don't want to bother. that is why contracts are broken all the time, but this does not make it legal to do so. If you have emails and satisfy all the requirements of a contract, it could be enforceable.
For example, people speed all the time, does it make it legal to do so? If you are in the flow of traffic doing over 10mph, the same as everyone around you, and you are pulled over, You can't say, they are all doing it. You are the one they are enforcing the law for.
hope this helps.