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RedThere's a lot of brokers out there who will sell cards they don't own. They know they're available for sale someplace else and if they can flip them for a profit that's what brokers do. The only legal issue will be if they take money for a card and don't deliver the exact card purchased.
If somebody knew your chair was for sale, and they knew somebody else who needed a chair, then they might try selling your chair to the other person. They would then come back to you, buy the chair, and then deliver it to the other person. If money changed hands and the chair was no longer available it would be expected that things would be made right. If not, then there's an issue.