Never Buy Coins From Chinese Sellers
When you buy coins, look to see where the person is shipping the coin from. If it is China or Hong Kong, do not bid! No matter how appealing the coin looks, no matter how great a deal you think you are getting, no matter how good the sales pitch is, do not buy any coins from China or Hong Kong based sellers. Following this one guideline will prevent you from becoming a victim 90% of the time, since 90% of all fake coins on eBay come from Chinese sellers.
Never Buy Coins From Sellers Who Buy From Chinese Sources
Before you buy any non-PCGS or non-NGC coin on eBay, always check the seller's feedback to see who he does business with. If the seller has done transactions with China and Hong Kong based sellers, do not buy coins from him!
This might seem to be a no-brainer, but you'd be surprised how many dishonest people do all of their buying and selling from the same eBay account. Check the feedback to see who they buy from by clicking on the "Feedback as a Buyer" tab on their feedback page (which you can reach by clicking on the number in parentheses next to their eBay handle). If they have ever bought coins from China, even once, do not buy from them! Most Chinese counterfeiters sell off-eBay after the first purchase.
Never Spend More Than $50 for a Raw Coin
A raw coin is a coin that hasn't been graded and authenticated by a reliable grading service such as PCGS or NGC. Even though a coin might be in a plastic holder of some kind, unless it is in a top grading service holder, it doesn't offer any protection against counterfeiting.
It is very difficult to judge the authenticity and quality of a coin based on photographs alone, especially the less-than-expert photos often seen on eBay. And even if the photo is great, who's to say that you will actually receive that coin? Counterfeiters are known to use genuine coin images but then send out fakes. Avoid becoming a victim by never spending more than you can afford to lose when you buy raw coins on eBay.
Don't Fall for Stories and Always Get a Guarantee
A favorite practice of fake coin sellers is to claim that they aren't a coin expert, so you should judge the photo for yourself. They might claim that the coins belonged to a recently-deceased aunt or that they were bought at an estate sale. What these claims have in common is that they usually disclaim responsibility for the authenticity or grade of the coin, selling the coin "as is."
Never buy raw coins on eBay unless the seller will guarantee their authenticity! Don't be a sucker for tall tales, thinking that maybe you'll make a score. More often than not, it'll be the seller laughing all the way to the bank rather than you, the buyer.
Always Pay With a Credit Card
I thought about making this tip #1, but I put it last hoping you will remember it even if you forget everything else: Always pay for coins on eBay using your credit card! Even if you use PayPal, use a credit card to fund the transaction.
The reason is that under current U.S. banking laws, you always have the final say on that money when you use credit (not debit cards, either; credit only!) The eBay and PayPal Buyer "Protection" plans are weak, and I hear complaints all the time about how difficult they make it to collect on a claim. But credit card transactions can always be charged back as a last resort. Even PayPal cannot override a credit card chargeback!
If you pay with a credit card, the final power is always in your hands.