A non-Sprint customers looking to unlock a 3rd party-purchased Sprint phone may be able to do so under certain circumstances. Non-Sprint customers will need to provide the account number or full account name last associated with the device. If this is possible, Sprint may be able to unlock the device or provide an MSL Code. The device would need to be eligible for unlocking by having met all financial and account requirements.
Sprint strongly encourages you to contact Sprint about a device's unlock eligibility before purchasing through a non-Sprint channel as Sprint cannot share any information regarding the previous account.