There are many, many fakes and frauds of coins out there. Unless you are very careful, you can lose a lot of money in a very short time. Any websites that I link to on this page are good are far as I am aware. It does not consist of an endorsement of any particular site as being problem free. There have been a variety of instances where high quality fakes have even gotten past very reputable dealers. To my knowledge the sites and coin pictures I am linking to have a good reputation in the industry, but as with anything, buyer beware!!!!
Some tips:
Study coins from a legitimate source first before buying any other coins. One good source is from Vcoins. It has a very good Code of Ethics and is fairly well trusted by those in the industry. Another great place to look at examples of coins is CoinArchives which is a repository of coins previously featured in major numismatic auctions. At both of these sites you can search by particular people or cities. Another good idea is to get a few reference books first. David R Sear is respected in the industry and has written a variety of very good books that I like. There are a bunch of good resources on my references page.
Ebay (and other auction sites). LOTS of issues with fakes and frauds according to the experts. Be very careful, even if you think that you know what you are doing. If you feel something is wrong with any coin, I wouldn't take the chance at all. Personally, I would never bid on an item where the bidder information is kept "private". Fakers will use this so more knowledgeable people cannot warn away prospective bidders. Be aware that even somebody with a good rating can be a bad seller. Sometimes bad ratings are hidden beneath a pile of good ones. Sometimes the good ones are there from legitimate buyers. Sometimes good ratings are left by people who don't know that they've bought a fake coin. Sometimes ratings are built up by rings of sellers buying from each other just to run up the positive ratings. Even a 100% positive rating doesn't mean that it is a reliable seller.
From this Reddit post, here are 3 sites that list known fake sellers on Ebay:
http://augustuscoins.com/ed/fakesellers.html
http://www.lateromanbronzecoinforum.com/index.php?topic=1218.0
https://www.forumancientcoins.com/fakes/ (need to create free account)
There is another type of coin out there that is between authentic and fake. It is called a tooled coin. It is a coin that was authentic that somebody has worked with to either assume the style of a more expensive coin, or to change a coin's actual surface to look better than it really was. An example in my collection as commented on by Curtis Clay, "Your coin looks authentic, but tooled, especially in Elagabalus' hair, Maesa's hair and bust, hair and clothing of Nemesis." (link)
Go through some of the fake and fraud sites out there to familiarize yourself with what to look for:
Calgary Coin. A very good and in depth site that shows how fakes are made and what to look out for. I would DEFINITELY go to this website and go through the pages before every considering buying a single coin.
Forgerynetwork. A new site that lists coins that are fakes or suspected fakes.
Do not take this page as the only warnings! These are just a couple of the most important tips
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