I don't see how anyone interprets this as scamming, unless the op knowingly told the buyer that the phone was for his carrier. If he never stated that he had proof that it worked on the buyer's network, then the use of the property is up to the buyer.
It's kind of wrong that you didn't tell him, or that he didn't ask. However, I don't see how you did anything wrong. Even if it was for Sprint or Verizon. Depending on the model of iPhone, some come with CDMA and GSM or just one of them.
So, it is possible for some iPhones to be flashed to other networks, depending on the location, network, and model.
For future sales; be more informative.