Employment Law Questions? Ask an Employment Lawyer.
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You can threaten to contact the police on the ground that the picture has been stolen from you because they are not giving it back to you. You can also file a lawsuit in small claims court for the value of the poster. Either of these options will drag the book store into legal trouble that they have every reason to avoid. The threat of either (or both) should be enough to provoke a quick response. If it is not, then you have these options at your disposal.
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