Legal Question in Business Law in California

It depends upon what the LLC operating agreement says. You must have signed one when you formed the LLC. An LLC is actually a form of partnership and requires an operating agreement between the owners specifying how the company is to be run. Usually, one member (partner) isn't allowed to sell their interest without the permission of the other members because of the close partner relationship. And, even if the member is allowed to sell their interest, no one can sell more than they own, so the 80% owner can only sell their 80%. If the buyer wants your 20%, he would need to deal with you separately.