The phones are sold at a loss in hopes that people will buy pre-paid minutes in the months to come. What some savvy people (some middle eastern) have figured out is that they can disable the electronic lockout in them making them useable by various plans and then sell them for quite a bit more than they paid for them.
IOW, usually this sort of thing is free-enterprise (albeit an unethical flavor) in action and not terrorism.
1) Aware of the phone-reselling scheme and
2) Vigilant for real threats
This activity to me does not seem like a real threat, unless the reselling scheme were used as a cover for something else (unlikely).