With other phones, it's more likely that a code can be generated because the algorithm to generate codes has been figured out and someone is selling generators to the people on eBay, the market stalls, etc. It doesn't necessarily need to go to Nokia or HTC or LG or whoever.
With the iPhone, Apple is deeply involved as the phone asks Apple for its lock status each time the phone is first used/restored or when a new SIM is inserted. This means the seller on eBay is likely having to pass your details onto someone at the network operators, or even Apple themselves, who is willing to put it into the system for a bit of cash on the side (how that works without someone at the network or at Apple figuring it out, I don't know).
If you could, you probably should have gone through the actual network - they'd likely have done it more quickly