Bear in mind that you will come across two kinds of fake goods:
Run On, Third Shift or Ghost Shift Where a manufacture (or their staff) who Orbit subcontracts production to makes more of the products that required and sells on the excess through a gray channel. This may include slight seconds and the substitution of lower quality parts and materials, several people have mentioned missing sears and not using stainless bearings.
Counterfeit Goods bearing the manufacturers mark but with no resemblance to the originals. The cheapest headset at chain reaction is £10 so there is a profit to be made in re-branding them and setting them at £25.
As to why counterfeit bike parts and I'm speculating here but if you sell counterfeit UGG boots to 1000s of people sooner or later someone is going to take in interest bike parts, who knows.