That it's a sprint phone.
Where are you seeing these? If it's in the U.S. and its cdma its going to most likely be sprint or Verizon.
You could always get the esn and call sprint and ask. However, even if it's a sprint phone that doesn't mean you will be able to activate it. It may have outstanding charges that have to be resolved or may have been reported as lost or stolen.
If you could just tell us what you're up to we could help.