There is an huge number of fake Compact Flash cards being sold. Many are sold through Ebay or Amazon Marketplace. Most are under the Sandisk label as this is the best known premium brand.
Did you know almost anyone can sign up to sell on Amazon Marketplace? It's like Ebay but with fixed prices, it is not the same as Amazon.
Beware sellers with little or no history. Carefully check feedback, especially neutral feedback, for the words fake or counterfeit. If they have been caught selling fakes once the chances are there are a lot more customers who didn't spot the fakes, or were just relieved to get their money back.
If the price is too good to be true it almost certainly is not a true card. Sellers selling at close to half the price of the cheapest genuine retailers are probably selling fakes. They often use pictures of genuine cards, not the fakes they are going to send out.

How to spot a fake Sandisk card?
See http://tinyurl.com/pphfh for the best advice, although note that Sandisk no longer use a hologram on the package.
A quick check is that the top corners of the metal frame on the genuine card are rounded. If the top corners are angular it's a fake. The genuine retail article comes in a heat sealed blister pack in a smart box.
Sandisk don't help, by not showing all the variations of the genuine article on their web site. Sandisk will verify a card from emailed photos.