Original: Rowling was hesitant to sell the rights because she "didn't want to give them control over the rest of the story" by selling the rights to the characters, which would have enabled Warner Bros. to make non-author-written sequels.
Simplified: Rowling was hesitant to sell the rights because she "didn't want to give them control over the rest of the story" by selling the rights to the characters, which would have enabled WB to make non-author-written sequels.