I've seen so many people on ebay saying that they got an iPhone 3Gs but they dont want it, and then they sell it there.

How can they do that?

1st case) If I have a clean SSN (without any ATT Contract), can I buy one at 199, and then, never activate it without being charged?

2nd case) If I have someone with an ATT account, with another phone (not iphone), can they get it for 199, and never use it, nor get charged the data plan, and sell the 3gs?