Dealing w/ Online Fraudulent Sellers?

Nicholas French asks: "I have recently made a couple of online purchases, one was an Ancient Bronze Amlash ring from The Antiquities Company which cost me *cough* 'a lot of money' and was backed up by a 'guarantee of authenticity'. The other was a second hand Book Crowds & Power, via the Amazon Marketplace, which I have not even received regardless of the numerous emails sent to both the seller and Amazon. I have since taken delivery of the ring and had its metal composition tested...turns out it is actually brass, and not exactly Ancient either! When I approached the seller via email, quoting my money back 'guarantee of authenticity' I was told politely to take a hike. I am considering speaking to Trading standards but am not really sure where to start, so have added myself to the growing numbers of reported Fraud victims on Fraud.Org. Have any other Slashdot readers managed to retrieve their hard earned cash from these Fraudsters, or had any similar problems with large online's such as Amazon?"