Should Best Buy report back to Apple whenever a card is sold?
Many gift cards sold at retail have to be "activated" at the register (presumably to cut down on shoplifting); I don't know what this involves but there may already be some communication.
Well, in case of retail cards the keys are registered before shipping. You give the manufacture a set of registered keys, which are then printed on the cards. If done right you won't have any issues - that's exactly what people are doing with CD-Keys or Prepaid-Cards. However, if you mess up (can't always avoid mistakes) and have shipped invalid keys or may be the key-printer didn't work right, you have the customer send you the certificate card and you can refund him.
It's by far the better system than using just a algorithm-based genuine check, especially for things that directly translate into money, like gift certificates.
There's also probably a chance of collision, considering the volume of iTunes certificates Apple probably sells.