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If he has given you a painting as collateral, you have a right to sell that painting to recover what you are due. The problem is there is no real way to put any lien or attachment on the painting other than by written contract
with whatever auction house you use to sell the painting or by suing the art dealer and getting a judgment and getting a court order seizing the painting. Your possession
of the collateral is your actual lien on the painting, as that is a common law possessory lien.
Thus, you have the right to sell the collateral and you would need a contract with the auction house that states you are due the payment on the proceeds from the auction.
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