U.S. numismatists work with a highly developed grading system that classifies minute differences between grades. Although attempts have been made to impose U.S. standards on the rest of the world, most countries are perfectly happy with their own, simpler grading standards that they've used for centuries.
For the most part, collectors of world coins are not as obsessed with quality as are U.S. collectors. For example, collectors in Germany are more interested in obtaining as many different coins as possible, and whether the coin is circulated or uncirculated is less important. This means these collectors aren't willing to pay much of a premium to obtain an uncirculated coin when they're just as happy with a nice, high-grade circulated extremely fine coin.
There is a danger in assuming that foreign markets will support the huge premiums that Americans pay for the best-quality coins. Here's an example: In 1989, the U.S. market hit a cyclical peak. Coin prices were extremely high, dealers were making lots of money, and the economy was good. Slabbing (coin grading and certification) of U.S. coins was only a few years old, while the slabbing of world coins was brand new. Collectors and dealers from the United States began paying huge premiums for high-grade world coins, partly because money was loose and partly because of perceived rarity. Gem coins (MS-65 or PF-65) began selling above the values published in catalogs; anything better (MS-66 or PF-66 and up) brought multiples of the catalog value. Unfortunately, the only ones paying the premiums were U.S. dealers and collectors buyers in foreign countries thought the Americans were nuts. And it seems they were right, because as prices for U.S. coins fell, so did the premiums being paid for world coins.
