mmissinglink That is something I have been trying to do for over 20 years. I have a chapter in my new book devoted to that. But I can briefly summarize my advice to beginners.
My advice starts with reading everything you can about your target coin(s). How was it made - on what kind of apparatus? What is the metal? Then research current prices for that coin so that you know what is the correct price. Ask on forums like this if your target coin has been a target for forgers too.
Look at forgeries at every opportunity. At most coin shows dealers will have a few forgeries for examples - ask for help - most decent dealers will help you. Once you have seen most of the common forgery types you will understand what the fake types look like.
You do not need to look at every forgery - but you do need to look at every
kind of forgery made. Casts have a cast look - the specific coin copied does not change that look. Transfer dies have an altogether different look. Once you have seen all the different ways a forgery can look you will be able to avoid that look in the future. In this area go with your gut. I quite often look at a coin and my brain registers fake - it may take me a while to pinpoint actual reasons - but listen to you instincts - you will find you know a coin is wrong before you know WHY it is wrong.
Also look at originals of your target coin. Coins that are hand hammered, roller pressed or screw pressed will all look different from a modern coin struck in a closed chamber press. Get familiar with the different looks. That way if you see a coin made in a closed chamber press that was supposed to be made on a screw press you will immediately recognize the difference.
Then study grading of coins so that you know what each grade actually means and which types of problems cause prices to drop rapidly. Remember buying an over graded coin can be as financially disastrous as buying a counterfeit.
Finally you need to remember that not all counterfeits are worthless. Many are worth hundreds of dollars. On more than one occasion a person has come to me with a bargain they thought was real only to find it was a counterfeit. In a few cases, they have actually made money because I do buy counterfeits of the collectible type.
Once you are familiar with all of this you can look at a venue like
ebay fairly safely.
But like all of us when we were novices - I know you will still rush forward and learn the hard way. So here are some other cautions in your entry into
ebay.
Avoid dealers with low feedback numbers who sell rare coins. Avoid sellers in China, India, the Philippines and Malaysia until you know more.
Avoid brand new sellers - they can disappear faster than they appear.
Avoid sellers that use stock photographs.
Avoid coins that are TOO GOOD TO BE TRUE - they are fakes.
Avoid bargains - they do NOT EXIST.
Be patient - unless you are starting with a monster rarity (at a monster price) - the same coin will appear again. They are usually more coins than buyers for average material.
Begin with common coins at low values and make your mistakes on coins that do not cost you too much.
Ask questions - here and any place else. There are lots of older guys who are willing to share.
Being a novice does not mean being a dummy.
Being a dummy is not asking a question when you have one.