Because Roman coins were produced in large numbers and so the price of many Roman coins is low, there are less fake roman coins then Modern day coins which are often more expensive and rarer. However that does not mean that copies exist. Alot of Counterfeit coins sold on ebay originate from eastern Europe, such as Bulgaria. Many of the more expensive and rare coins are sold to tourists in the Mediterranean.
If you are an experienced coin dealer you should be able to tell the fake coins from the real ones.
If you are not an experienced coin dealer then you should consider purchasing coins from dealers with a high feedback score who have been selling Roman coins over a number of years. The sellers of fake coins are more likely to be weeded out this way.
One giveaway is the surface; ancient coins were hand struck and hence will have a smooth surface and a good sharp detail, while copies will be usually cast, which traps air bubbles in the metal that show up and leave the surface rougher/bubbly with fuzzy detail. This should not to be mistaken with a rough patina. Also, a seam might show up along the edge of a cast coin. The style of the letters is another giveaway; they are often more perfect on copies than originals, with the tips of letters like A, M or V for example coming nicely together which often didn’t happen on genuine coins. Genuine coins are often not perfectly centred , they show cracks, have missing chips around the edges, have patinas, bits of dirt or heavy encrustation and are of uneven thickness (all qualities that fake coins often do not have). You simply need to handle a bunch of these to notice the differences.
There are is a number of websites that you can find which will show copies and you can learn how to spot them. However, this should not worry you as these are not rare coins! You are in fact more likely to run into a fake of a rare modern coin than a cheap Roman coin. The best thing to do is study lots of coins (pictures and actual coins) and buy from dealers such as those on ebay that have been around for a while and sell ancient coins.
As an example, compare the two coins in Figure 1 below. The Gordian II piece is worth about $2000, but it sold on eBay for over $150 and 11 people bid on it. The seller had a feedback only 2 and sold no other ancient coins before (cheap or expensive). This is usually the case in all such auctions, so beware of that. Real dealers will have feedback into the thousands and be constantly selling all sorts of ancient coins. Also, if this coin was not an obvious fake, its price would be much higher because people who know what they are buying would bid on it. Here is a list of genuine Ebay Roman coins sold by over 70 genuine ebay Roman coin sellers.
Figure 1: Fake coin of Gordian II (left) and a genuine coin of Maximinus I (right). Notice the nice straight and easily readable letters on the fake and that the letters are not as straight on the real coin. The fake is ’too perfect’.
Purchasing Roman coins
Sellers of ancient coins will often use terms such as common, uncommon, scarce, rare or very rare to describe a coin. Just because someone describes a coin as rare does not mean that it actually is. Unfortunately, some sellers will attach this label to ancient coins to attract novice collectors who do not know better. Does the person explain why the coin is rare? Does he give it an RIC number and explain the variety? If you are new to collecting ancients, you should not jump on a coin just because is it described as rare, especially if you are not familiar with it. Scarce is a more accurate description I find, because less sellers would use it to attract a buyer. Eventually, if you do build up a collection and are aware of many coins from catalogs and eBay, you will be able to tell yourself if a coin is uncommon, scare or even rare. Also, truly rare coins (that are desirable) will not sell for much less than $100, and often for much more. Hence, a coin of Constantine described as rare with an asking price of just a few bucks is very unlikely to be so. There are sellers on eBay who claim that their coin is rare, or they have only seen one in 10 years, but there is another one just like it being offered on eBay by someone else!
So, to summarize. Buy coins that you find interesting and that appeal to you. No two coins of the same type will be exactly the same; choose the one that you think is a better value for your money. Look around to see if a nicer looking coin might be also affordable if that is what you really want, and hence save yourself the trouble of replacing it later or regretting that you bought it in the first place. Do not worry about rarity too much!
If you show any of your ancient coins to a non-collector or a modern coin collector, they will be totally amazed that you have something that old and will think that they are all very rare (or fake!). Keep in mind that many Roman coins are not rare. To give you an idea, quoting Van Meter’s Handbook of Roman Imperial Coins ”...scholars estimate that the mint [of Rome] was striking around 400,000 denarii per month from 85-87 AD, but then production surged to over a million per month the next year, and peaked at 2.5 million per month in 92 AD.” This is far more than many early US coins for example, which are only 150-200 years old. Yes, many were lost to time, but many still remain, and remember, there are fewer collectors of ancients than of modern. Over time, as you get more coins, some of them will be more scarce than others, and you will have a sense of satisfaction owning them, but to another collector who has a different interest, these coins would not be worth the same as they are to you.