Another Craigslist story

Hi everyone!
First of all excuse my English and thanks for the advice.
In May of this year i did a post on Craigslist trying to trade my iPhone 6s plus black 128gb
For a gold or silver iPhone 6s plus 64 or 128gb, basically I wanted a white iPhone right.
Well it took maybe a day an I ended up with a 6s plus 64gb in new condition and almost a year of warranty, we even meet at the Apple Store to check water damage, iCloud lock, etc etc, everything was perfect, seemed like a good guy the person whom I traded.

Yesterday I was selling the phone to a friend to buy the 7 plus. I did factory reset and told my friend "here, all yours"
Here's when it gets tricky
My friend was going thru the iPhone configuration and was starting the phone as new iPhone but he couldn't pass by this "IBM username and password" screen.
then on the bottom screen there was a sentence saying this iphone is owned by IBM so lots of things when the thru my mind.
-Maybe the guy reported the phone as lost
-the guy worked for IBM and didn't tell me
-is the phone bricked now?
So I gave my friend his money back and apologized for the inconvenience, I felt bad because of the misunderstanding.
Soooo I took the phone home and it did allowed me to restore the phone without any problems. That being said I'm pretty sure it won't let me reset the phone and configured as new without this username and password.
So now I'm Soooo Confused.
What should I do?

Thanks in advance for your suggestions and advices guys

 

I didn't know it was stolen just because we both were OK meeting at the Apple Store.

How did I restore it? It had an option to setup as new or restore.