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This is a common numismatic saying that means you should learn about a coin before spending money on it. Consult current pricing guides, like the printed Handbook of United States Coins or the web-based Professional Coin Grading Service, before committing to any purchases. These will allow you to find and compare the current market price of different coins and grades.  For foreign coins, look at nation-specific guides and databases like Numista. For ancient coins, consult specialty books like Ancient Coin Collecting. If a deal sounds too good to be true, it probably is. If a coin is underpriced, check to make sure it’s real and hasn’t been buffed or polished to hide faults. If you found it at a garage sale, flea market, or similar establishment, the seller may not know the actual value of their merchandise, but specialty sellers and collectors certainly do. Coins are graded differently depending on the country and individual appraiser, but a good starting place is the Official A.N.A. Grading System for United States Coins. Here, coins are ranked on a scale from 0 to 70, with extra points being given to uncirculated coins. Letters are added to denote quality, like MS for Mint State or VG for Very Good. In this system, the highest quality coin is listed as MS-70.  In general, US appraisers are more lenient than UK ones, so be aware that a perfect coin in one country may be considered flawed in another. Some people overgrade their own coins to make them more market friendly. To avoid being conned, make sure to double-check all coins using an official “by the book” standard. Remember that grading, even by a professional service, is subjective and that grading standards change over time. For serious collectors, purchase both a low powered and a high powered magnifying glass. This will allow you to look for small imperfections and signs of forgery, like coins with incorrect fonts or askew visual elements. Pay close attention to shiny coins, as details may have been buffed away to appear glossier. A portable electronic scale is an important investment for collectors making expensive purchases. Weighing a coin and comparing it to collecting guides can help you spot counterfeits made from cheaper materials. Additionally, weighing a coin can help you figure out its melt value, or how much it would go for if melted down into its raw materials.

Summary:
Buy the book before the coin. Avoid underpriced coins from professional sellers. Learn how coins are graded. Buy a magnifying glass. Buy a scale.