You would likely benefit from some tips on how to buy US coins if you are not too experienced in distinguishing between genuine US coins and fakes. There are currently more counterfeit old US coins flooding the market than genuine ones, and it is important that you can spot these. Fortunately, there are several 'tells' that you can use to do so.
Forgers Plate Base Metals
Forgers do not use real silver and gold to fabricate coins - obviously! They take a base metal such as lead or tin, add a copper layer and electroplate that with silver or gold. Some add a layer of nickel after the copper when electroplating gold.
However, the point is that silver or gold coins such as the Silver Eagle and Gold Eagle can easily be counterfeited to look like the real thing - unless you know what to look for. Once a surface has been electroplated, it starts to degrade over time. If you use a glass or loupe to examine the reeding or milling on the edges, then you may see areas where the plating is flaking off. The same is true around other raised areas, particularly the sharp areas such as the designer's initials.
Design Features Are Missing
Speaking of designer's initials, these don't always appear at the same place with each coin, and by learning where they should be for the rare coins you can easily spot a fake. Sometimes it is missing altogether, so look for the designer's initials and make sure that a) they are there, b) they are correct, and c) they are in the proper place for the date of the coin.
Check the Reeding
The reeding is the knurled edging that most coins have, and is known as 'milling' in the UK. Not all coins have reeding, but they do so from dimes up to some dollar coins. Not all dollars are reeded, so make sure you know which coins are. A lack of reeding could indicate a forgery.
Weigh the Coin
Those that regularly buy US coins know that coins have a standard weight. Different issues of the same coin might have different standard weights, but it is a property that can be used to distinguish between the real thing and counterfeit coins. By learning what the weights of rare US coins should be, you can help to avoid fakes when you buy US coins. Wear does not significantly affect the weight of coins.
Buy Certified US Coins
Coin certification and grading bodies such as PCGS, NGC, ANACS and SEGS will certify the authenticity of coins for sale. By using one of these bodies to certify the coins you intend purchasing, you can be sure to buy US coins that are genuine.
If you ask the seller for certification and they refuse, then you can be sure the coin is a fake. If they say OK, but you must pay, then you can be as sure as you can be without the actual testing that it is genuine.
These are just some of the steps you can take to make sure that when you buy US coins they are the genuine articles and not some cheap fake. Most of these are coming from the Far East: mainly China and Hong Kong. Don't be one of the mugs that get mugged!