If you can't deliver the car with its title, you will have to give all the money back, just to keep yourself in civil court for breach of contract, and not also be charged criminally for fraud. You're correct that it won't do you any good in the civil court. But you won't necessarily be able to recover damages from a car scammer in civil court, either. Yes, you win, but how do you convert a judgment in your hand into a car you can drive?

Perhaps it would be best not to reason this one out with a car analogy. It's distracting from the main issue.

Some people are claiming that Dr. No-I-won't-go was trespassing after being told to leave. That is not the case. That would be equivalent to a landlord evicting a tenant with a valid, paid lease. While the landlord also could not force someone out just by refunding their prepaid rents, there is no circumstance that would allow the landlord to evict a lease-holding tenant (if any even exist) without also refunding that prepaid rent. So as long as that hasn't been paid, there can be no trespass.

Furthermore, the tenant would be ethically justified to refuse to vacate the property until the refund had been paid, because holding possession of the property is the only leverage he really has over the landlord. He could go to civil court and petition for it, but if the landlord is judgment-proof or employs a good lawyer, that money may be out of reach for years.

Even if there were a law or rescission clause that allowed United to refuse service, without a cash refund, there is no possibility for criminal trespass. Without a crime, there is only a civil dispute. And there is no reason for the cops to be involved in any way in a civil dispute, especially not to be choosing sides in it and beating someone up.