Hello 'gcru',
Truth is you'll find both authentic and fake in just about everything. The most important thing is to know something about the product you are buying. Sometimes price can be a clue, but not always. A seller might be clearing out her late aunt's stuff and sell authentic items well below their worth. Or a valuable watch may be listed in auction with a low start bid, but be over $1000 in a few days.
If you think the item is fake there is a report button on every listing. And if you want to see ebay's official blurb about authenticity, the following link tells all about it. Third item down on the 'not allowed' list, - replicas:
http://pages.ebay.ca/help/policies/replica-counterfeit.html