Jim told Huck that he had run away from whom?

Jim ran away from Miss Watson in the book because he heard that she was going to sell him down south. She had promised him that she wouldn't, but she needed the money since he was worth a lot. After hearing the news, he fled because in the south the plantations treated the slaves poorly.

The further South a slave was sent, the harder they were worked. The cotton was shipped North, so the further South you were, the more shipping cut into profits- and the only way to compensate was to grow more cotton as economically as possible. That is why, to this day, "selling someone south" is a way of saying "betraying them".