some people seem to have some misconceptions to be honest. Yoshino sell the kidney not only because she was angry at being insulted, but she have to actually prove that she had some value while living with him and shut him up. like her grandpa had said, she had no choice but to stay there for a year, which is why she had to do something to prove that she is not that innocent and fragile type of girl, whom other characters can mess her up anytime and anywhere.