Condor - yes I am an attorney, but rather than personally attacking me, please focus on the facts we are discussing. IMHO, there was a contract formed, both by words and by action. Both parties agreed on a price on a specific identified gun. Money was sent and clearly accepted.
That the seller didn't know the actual value of the gun is immaterial, and such mistakes happen all the time in business, and are no basis for undoing a contract.
Imagine that Person A sells a gun to Person B for $1000. Turns out the gun is worth $1100. Is that a "mistake of fact" sufficient to undo the transaction. I say no. I don't think the conclusion is any differnt based solely on the amount of the mistake made by the seller, as well. (Although a large error in valuation could be used as evidence of mental incapacity by the seller, etc., but that would be a different basis for voiding the contract.)
If the answer was "yes," business could never be done by anyone because no sale would ever be final since the "exact value" of something is always debateable. Imagine a different scenario: A person buys a new car from the dealer, the buyer pays the asking price, the dealership deposits the money into their account, but, just as the buyer steps into the vehicle to drive away, the salesman returns the money and says "sorry, we are undoing the sale, I just learned that this car is worth $1000 more than what I sold it to you for." Who's to say the salesman's new value is correct? What is to keep the salesman from undoing the deal multiple times, based on getting new value information each time? I think the economy would grind to a halt if people could undo contracts in such an arbitray manner.
Interesting discussion about contract formation, but like I said, it is academic at this point since DrBob has thankfully recovered his money.
JMHO