[duck9191]

if its attached to someones account it means that its still active on someones subscription, or they still have a pending balance with sprint. once that subscription is canceled, or the balance is paid even sprint will release the esn from the subscription and when you try to activated it the system will not see it as a "in use" esn. normally people just stop paying their bill and think they can sell the phone.

if it was flagged stolen the rep would have gotten a different error. either way your screwed with sprint but will be ok on metro or cricket by re-flashing like others have said since they don't care that the esn is in use.