Kim, you will also discover third parties selling OEM parts for our cars. Some of these OEM parts are indeed what Porsche puts in their boxes and sells for far more, such as Zimmerman brake discs.
However, I've purchased "OEM" parts from a company on eBay called Deutche Parts USA that claim they sell OEM parts... and they might be but they're not nearly as sturdy as the parts I could buy from Porsche so it's not worth the discount if you have to replace the part less than a year later.
In my opinion, there are some parts on our cars that should not be replaced with Porsche parts... the lower front control arms are one example. The bushings wear out prematurely. The remanufactured ones from Vertex last a lot longer because they are not rubber but made from a harder, more durable material.
Same thing goes with the oxygen sensors.
www.oxygensensors.com sells Bosch versions with the correct connectors for 1/3 of the price at my Porsche parts counter.
Of course, anything anywhere is 1/3 the price of my local Porsche parts counter. They price gouge every chance they get!
Best thing to do is buy what others have bought and used for a long period of time and found to be worth the money paid, regardless of price.