'Selling one's soul' means to do anything, no matter how immoral, to achieve an objective. It basically means not to do anything to reckless for a few points on a grade. For example cheating.
I think "sell your soul" means to do anything no matter how extreme it is, so if you're thinking of this in a school way (on a grade), this might mean to not do anything to extreme (like cheating) just for a few points on a grade. I hoped this help, and I hope this is correct lol.
