Why do people buy perfumes on eBay and then complain about them being "fake"?

The point of ebay is to get a good deal and if people are listing fake items they should actually say in the listing that it is not a genuine item. many people on ebay are genuine sellers and sell genuine products - it is just a few people who list items as real which are really fake. they ruin it for the people who are genuine. Its not that they are stupid they just expect sellers to be truthful in their listing.