Stay out of it. You shouldn’t report to either parent. It’s nothing to do with you. I find it a bit out there that dad wants to sell mom’s bike, but again it’s between them two. That’s why I recommend you don’t live with parents in adulthood. Less you know about your parents marriage better you are. Pretend you are busy, have headphones on and don’t hear them when they tell on each other. Don’t tell him when mom comes over etc, if he asks say it’s not your business.