I am currently an iphone 3g user on AT&T. would it be possible and legit if i were to buy a used iphone 3gs from a user who perhaps wasnt happy with their purchase and decided to sell me theirs. Could I simply stick my current AT&T sim card from my old iphone 3g to the new 3gs one and use it? Not sure how this would work. Cause im sure there will be plenty of iphone 3gs for sale online within the coming weeks.