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identify7If you want an unaltered coin, a good source is the U.S. Mint.
Coins are artfully altered by methods including dipping, putting them in potatoes and heating them up in the oven, and the employment of multiple, sometimes exotic techniques designed to produce an even luster, interesting or attractive toning, brilliant shine, and other surface characteristics which will motivate sales.
If you are knowledgable and careful in numismatics, you have a chance of getting what you pay for. But original condition unaltered coins are traded among knowledgable collectors, generally. These collectors typically have areas of specialty. The range of specialization covers seeking altered coins (such as hobo nickles) through preserving historical examples of US coinage which include cents and half cents from the 18th century.