There are many points to cover regarding the finding and winning of ancient Roman coins on Ebay.  Without a doubt, the most important point is whether the coin is an authentic ancient coin that was stamped during the reign of Caesar, Caligula, Constantine or the like.  Is it the real thing made by romans and their slaves centuries in the past?
          Regarding coins, Fraud is an attempt at selling something as authentic without it being so. It is a lie.
          Ebay is a fair market platform where dishonest and despicable sellers might seek an opportunity to steal and cheat.  I believe that Ebay is doing as good a job as can be expected while trying to maintain a free market environment.
          Reid Goldsborough at reidgold.com, in the article titled ‘Coin Fraud', but linked on a previous page as 'How to Avoid Coin Fraud on eBay' states ‘The FBI, on the other hand, has contended that the figure is much higher. As a part of its "Operation Cyber Loss" project, it determined that the rate of online auction fraud is about one in a hundred, or 1 percent. This is a very high rate of fraud, a whopping 400 times higher than what eBay contends.’.  I must say in defense of eBay, that the FBI was concerned with 'online auctions...', not eBay in particular.  As often is the case, the truth most likely lives between the two data.
          I have never received a counterfeit coin that I am aware of, and only once have I failed to receive the coin I won.  In this case, I failed to follow my own guidelines.  Do your research, and findout who is, and is not, a reputable, and trustworthy seller.  Don’t let counterfeits scare you off, or stop you from enjoying the wonderful feel of an ancient coin in hand that might have been spent, seen, or touched by history and history makers.  Rg.ancients.info is a great place to begin your education.  Knowledge emancipates and empowers more than any other single thing.
          Once the coin has been determined to be authentic, you must figure out what the coin is worth, what you are willing to pay for it, and how to increase the chances that your bid will be the winning bid.  First set the price you are willing to pay for a coin, and stick to it.  I mean this.  I try.
          It is important that you know how to search for ancient coins on Ebay.  Coin finding strategies, for my part, are important and add to the fun of collecting.
          Finding the coin you are seeking can be a treasure hunt.  If you are lucky, you might find a coin poorly listed that gets little attention.  In this case, a low bid might be enough.  I have acquired coins at a 10th of their market value.  Not often, but a few times, and with that said, be aware that a 10th of the market value is a third, or half, of what you might be able to get if you try to sell it.  That is a profit, but are you going to sell it; most likely not.  Those who successfully, and honestly acquire coins to sell at a profit do so by way of years of experience, broad knowledge, networks, and finacial resourses that do not describe me or the average collector. More power to them.  They make it possible for amateur collectors to build collections worth handing down to their kids, family, and friends.
          I started collecting in 1957. While building my collection, I always bought coins with the intention of never ever selling them. Never!!  Ever!! With that said, I confess that I sold 90% of my collection in 1965 in order to finance my move out of my parent's house, pay rent and deposit, and begin my university education; and be on the beach in order to surf.  It took decades to get over my feeling of loss., but I did; for the most part. (It is wiser to prepare for your next mistake, than brooding over your last.)
 

My strategies and bidding on eBay:

          How should a collector bid on a coin?  The most widely know strategy is to open a tab to view a listing by way of constant refreshing of the page, and have another tab with the same listing, but with your greatest bid ready to submit. You watch the progress of the listing on the first tab, and click you max bid on the second tab. Clicking in your bid on that second tab is about timing.  With that said, it is very important to have a very fast computer, or two, and, even more important, to have an internet connection that has a very fast ping, and delivery.
          I seldom pay more than than the market value for coin.  However, desire is very influential. When I find ‘that’ coin, and, if 'that' coin is in a condition that will increase the average quality of my collection, my desire takes flight.  I’m usually willing to go as high as 25% over my estimation of it’s market value.
          I often bid on coins in poorer and worn condition for many reasons.  They are rarely counterfeit, less expensive, and show and convey the actual touch of history in their wear, patina, dirt, and scars.  For years, I carried a dupondius issued by Hadrian in my pocket as a talisman.  It is worn smooth, can only be identified because Hadrian’s profile is so easy to identify, has the goddess Moneta on the reverse, and is beautiful to look at, and a pleasure to wonder about.  It is a wonderful large chunk of history that only cost me $8 in a coin store in San Francisco.
          So how are the prices on Ebay compared to the prices you might expect in the larger world market?  I believe they are at fair market value for the extremely coveted coins in gold, or the very rare favorites in or near uncirculated condition.  The popular and common coins are bargains, but not at an insignificant cost for many of us.  I’m talking about a coin like Caligula’s ‘Vesta & Livia’ issue that is very common in all conditions, but so popular that bidding can sometimes send a coin out of the bargain category.  But for me, the real runaway bargains are the coins of the 3rd and 4th centuries AD.  These coins are very old, often in great condition, common to the extreme, and filled with history.  Emperor Probus, Aurelianus, Constantine the Great, and Licinius are fine examples where coins in nearly mint condition can be acquired for the value of a dinner, or a tank of gasoline; and not nearly as fattening or polluting.

          Here are my guidelines for judging the authenticity, value, and significance of a coin, and how to bid in a way that increases your chance of winning the auction:

Authenticity:
          Everything about coin collecting orbits around authenticity. Above all, the experience, reputation and credentials of the seller is a strong indicator. An Ebay seller’s time with Ebay, feedback percentage & number of sells is a good indicator to judge honesty and ethical conduct. It is no easy job selling to hundreds and thousands of individuals, and maintain a percentage above 97%. Make sure that a guarantee of authenticity is stated. NEVER bid on a coin when the seller says they can not guarantee its authenticity. This is against Ebay rules, but not aggressively enforced by Ebay.  Ebay rules state: 'Sellers should make sure that their items are authentic before listing them on eBay. Sellers may not disclaim knowledge of, or responsibility for, the authenticity or legality of the items they offer in their listings. If a seller cannot verify the authenticity of an item, it can't be listed on eBay.'; (Statement located at LINK). I see many coins listed with this statement, and move on immediately. What clients say about a seller is extremely valuable. And what is written about the coin you are considering is also valuable. As you read what collectors and experts have said about a coin, you will start to gain confidence that you understand if it is authentic or not. Is the coin valuable enough to bother counterfeiting?

Value:
          Determining the value is where the fun really takes off. As you do your research, you will learn about the coin’s history, popularity, significance, and, of course, value. There are many ways to determine what a coin is worth, but to my mind, the best source of information is Wildwinds and Forum Ancient Coins. Both Wildwinds and Forum Ancient Coins links to archives and sellers with sound information, and ethical standards of practice. They are easy sites to use, educational, and are an all around effective tool. I have gotten lost in curiosity and pleasure studying one coin and then another; following links and enjoying the hunt; the education.
          Reading about emperors and events; heroes and villains; watching videos, movies, and TV series about the history that is the coin in your hand is satisfying, and very educational for the kids. Another possible source for value of a particular coin, and a great source for values and images of coins in general, and books, is Frank Robinson’s auctions. Links to these site are on most of the pages of Vrbs Roma.

Bidding:
          I don’t have a lot to say on this point. Most people already know how to ‘snipe’ an auction. Sniping, described above, is to place a bid in the last few seconds of an auction so no counter bid can be placed. Often a swarm of bidders are doing this, so good luck.
 

Good luck, and happy collecting!
 

PS:
          Books are a great way to gather information about coins and emperors. Wayne B. Sayles & David R. Sears are two of the most respected authors on coins, emperors and history, and Frank S. Robinson's book on collecting coins is an insightful and interesting read with very good advice for beginners.