If you see something fishy let your friends know. If you accidentally buy a fake, which is bound to happen from time to time, learn from the experience and make sure it not put back into circulation unless it is clearly and unalterably marked. If you can't think of anything else to do, contact me with your story. I don't expect everyone to make fraud detection a part time job, or mount a horse like Paul Revere. But those who do nothing when they see fakes contribute to the problem.
Every little bit helps.
Whether effective or not, I believe the eBay makes a sincere effort to prevent fakes. I don't have any experience or knowledge of amazon's efforts in the area, though I did buy a CD from amazon last week and it got here on time. I am aware of a few cases. I do believe, however, that the major auctions like Sotheby's, Christy's, Mastro, Leland's, Lew Lipset, to name a few, do good jobs at authentication. As a collector, I would buy from any of these places.