I had a similar situation...
Recently I switched from the HTC M8 to the Nexus 6.
When I originally got my HTC M8, a friend at work unlocked it and rooted it for me so that I could use apps like TiBu snd Ad Away.
About a year later, I decided to switch to the Nexus 6, and sold my HTC M8 to a Mobile phone reseller company.
I got a message from the company that they weren't able to activate the phone on Sprint, because it was unlocked.
After a few emails back and forth, the reseller rep I was working with was able to 'call in a favor' and get the HTC M8 added back into the Sprint database, so that it could be activated.
I decided to sell my HTC M8 to a Mobile Phone reseller.
When they got my phone, they were unable to activate it on Sprint because the phone showed as 'Blocked' in the Sprint database. When I told the Mobile phone reseller rep that the phone had been rooted and unlocked, he figured that was the reason that the phone was blocked in the Sprint database.
After a few emails back and forth between the rep and myself, the rep was able to 'call in a favor' from a coworker of his, and get the phone cleared in the Sprint database.