Scammers are everywhere, even in the high-end CPU market. An unsuspecting buyer paid hundreds of dollars for a Core i7-990X six-core LGA1366 processor, only to end up with a badly-done fake. The fake chip is a worthless LGA775 Pentium, on which the IHS markings of a Core i7-990X have been etched. Intel and AMD put tiny windows on their retail processor boxes so buyers could see these markings before breaking open the company seal.
The thread where this fake was reported can be read here.
In the case of the i7-990X shown in the Intel forums, anyone knowledgeable about processors would know it was not genuine, as the chip is in the wrong package (ie the wrong size), and fits the wrong socket. Unfortunately, not every consumer is as knowledgeable as most who frequent this site, so fakes like that can easily be sold on eBay and other outlets for a huge profit. It's unknown exactly which processor it was originally as the user who reported on it had not yet had a chance to test it, but one thing is for sure - it is a socket 775 chip and not a socket 1366, which it should be.
As a final thought, when buying parts (new or second-hand) from an unknown retailer, or through a private sale (eg on a forum where the seller is not known) you may not have any options to recover the cost of your investment. On the other hand, buying expensive components from large, established retailers usually means you will get genuine parts that they purchased either from the manufacturer or from an authorized distributor, meaning a far lower chance of a fake product slipping through, and a far greater chance of getting either a refund or replacement if your goods do not work as advertised (ie fakes or faulty goods). The manufacturer is under no obligation to help you if your goods are fake.