Htc one help!!!

[AC Question]

Okay I tried reading some stuff and have no clue, I am a total newb at this. I have NOT rooted my phone or put a custom ROM on. I stopped using the phone like 3 months ago when I upgraded, and today I decide I want to sell it but when I try to do a Master Reset the bootloader says "Tampered" and "Locked". I do see the it is S-On. I have no clue what these things means, but this is what I read when I was doing research. I did the System Updates, but again, did nothing custom personally. I need to hard reset the phone, please help!