But are they real? How can they be so cheap if they are so old? These are common questions from new or non-collectors. First, just because something is old does not mean that it has to be expensive – lots of new things are expensive (like one of those brand new proof silver dollars with zero years of history). The Roman Empire, as you may know, was pretty large and so they needed to make a lot of coins. Also, there are fewer collectors of ancient than of modern coins; this makes the demand for them lower. These factors of supply and demand keep the price very reasonable. And yes, most of them are very real.
Since common coins sell at low prices, it does not pay to make copies of them. Mind you,copies do exist, often of common coins that are sold to tourists in the Mediterranean and of expensive or very rare coins. However, if you buy coins from any respectable dealer,they will usually come with a guarantee (often lifetime) of authenticity. Eventually, you will be able to tell them apart – one giveaway is the surface; ancient coins were hand struck and hence will have a smooth surface and 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 (not be 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 centered, 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 ancient Roman. The best thing to do is study lots of coins (pictures and actual coins) and buy from dealers that have been around for a whilea nd 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 of 2! (two!!!) 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. This is experience. Again, you will not run into problems like that while buying a $10 coin.