What documentation should come with coins for sale on the Internet?

Coins purchased over the Internet or in person should always come with a certificate of authenticity. This serves as proof of the coin's authenticity, and it should also have information about the purity of the metal and the weight of the coin.

When purchasing a solid precious metal coin as a bullion investment, it may come with an assay card. The assaying process determines the purity of a precious metal, and an assay card provides the buyer proof of a certain level of fineness. Collectible coins often have a commemorative purpose, and the purity does not matter as much. Therefore, an assay card is not always included in a collectible purchase.