I have been collecting coins for a long time.  I mainly collect Korean and US coins.  I enjoy going to the coin dealer and see what they have for sale, or going to auctions to see how much people are paying for certain coins.  Then came the Ebay.  What a wonderful site.  You can buy and/or sell coins with clicks of a mouse.  Easy money, I thought.  Of course, where there is easy money, scammers are everywhere.  After few arounds with the scammers (i.e. dishonest coin dealers), here are few things you ought to look for a potential scammers when you are buying coins. 

1.  "Private" Feedbacks:  The seller is hiding his/her feedbacks from the buyers.  Ask yourself:  Why?  Too many negative feedbacks?  When you see that seller's feedbacks are "private", might want to rethink about buying from that seller.

2. Foreign Sellers:  Especially with Korean coins, beware of foregin sellers WITHOUT return policy.  ****Please note that NOT all foreign sellers are bad*****Whenever I buy Korean coins from foreign sellers, I leave feedbacks after I received submitted coins from NGC or NCS.  If the coins can be encapsulated, then good for me and good for the seller.  However, I had few occasions where the coins cannot be encapsulated and practically worthless.  Even with the return policy, you have to send the coins back via international mail and have to wait for 20 to 30 days for the refund to appear.  If you are buying from foreign sellers, look for their good feedbacks (AND no "private" feedbacks); and rest is up to you.

I hope this guide helps you and happy biddings!!!!!!