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Unfortunately, an honest mistake one party's end does not void a contract. However, if both parties are mistaken to where there can be no meeting of the minds, then the contract is void for lacking consent.
The buyer may be able to initiate a legal action for specific performance (requiring a party to a contract to perform the terms of the contract when he has refused to fulfill his obligations). Specific performance is usually only awarded if the buyer cannot get a substitute good or it will be very difficult to do so. This is seen in a case like the present, where the item is a one-of-a-kind such as a piece of art.
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If the "seller" had no authority to enter into the contract, then the buyer may actually have a cause of action for misrepresentation. In which case, the buyer maybe able to sue for any damages that resulted from the misrepresentation. It is hard to know what those damages would be, for example, if the buyer missed out on an opportunity to obtain a similar piece for a reasonable, then that could be the measure of damages. The measure of damages will be very factually specific.
Ultimately the burden of proving title will be left up to the seller (who will be using it as a defense) in a contract dispute or the buyer in a misrepresentation dispute.
Depending on the sellers representations, trying to sell a piece of art that does not belong to you, may also constitute fraud. In which case, the buyer may be able to sue for damages and punitive damages.