Where do you cash in your 1943 copper penny?

1943 copper pennies are extremely, extremely, extremely, extremely, rare. The first thing you should do is have it graded by a reputable grading company to make sure it is genuine. Assuming for some reason it is a genuine coin, you would get it auctioned by a large auctioning house specializing in coins such as Heritage Auctions.

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Also, be careful with terminology. "Cashing in" usually refers to taking a coin to a bank and converting it to some other form of currency rather than selling it. Banks are only obligated to exchange coins and bills at face value, regardless of their value to a collector on the open market.