Default Droid Razr M UnRoot

Hello Everyone! This is my first time here, so I may not have all terms correct. I am a rookie at rooting/unlocking. I made my first try with the Droid razr m xt907 awhile ago and was finally successful with the motochopper app. Then I discovered it was impossible to unlock the bootloader so it really didn't do much for me. yea, I know rooting still has some advantages but I just couldn't do what I wanted....

Fast forward to today. I just upgradd to the GS5 and want to sell my xt907. I would like to "unroot" it before selling, but am really not sure how or if it is possible. I do not have a stock rom, and am not sure what steps or software are involved so if anyone can explain to me it would be extremely appreciated. I can sell it as is, but it is not taking the new update available so that is a downside for selling.