LOL! They're not entitled to the car until a contract is signed. Whoever is threatening that they can take legal action against you because agreed over facebook, is CLEARLY not a lawyer.
Now don't get me wrong, it's bad form to agree to a price and then pull out after, but it's not illegal. Now if you had signed the contract, accepted the cash, that would be a different story.
But just because I agree to a price in exchange for a car with you, it's not legally binding until the transaction is complete.
Tell whoever it is threatening you, to go get bent. They don't know shti about the law because if they did, they'd know they don't have any legal standing to sue you.
Now the exception may be if there's a law in your particular state that actually does entitle enforcement but I've never heard of such a law, and it certainly doesn't apply here where I live. This is funny tho