Agree, I don't think you can lock the shutter with a CSM setting, but I am not a big user of custom settings and not totally familiar with them.
I don't know of any shutter speed lock other than using the "L" (shutter speed/aperature/focus area lock) button on back of camera while turning the Main Command Dial during Shutter Priority or Manual mode operation. But locking the shutter speed in this manner places "LOCK" above the shutter speed in the Top LCD Panel and an "L" in the bottom of the viewfinder (left side next to the Metering System indicator - far left), not the "X" you indicated. This type of lock is also cleared with the 2-button reset.
I hope you get the lock cleared and good luck with your sale.