We had to downsize after losing our 3000 sq ft home in the housing crisis 15 years ago. Like everyone at the time, we had a garage sale to get rid of the stuff that still held some value and raised almost $1k. Our 10-year old son, who to this day knows the value of a dollar, thought he'd stumbled across a money-making endeavor that would set us up for life. I gently told him that we had paid more than $20k to get $1k and he understood instantly that having too much stuff wasn't a good thing...
(He is now a professional musician at 21 and owns nothing except music/recording equipment and a second-hand car. I'm grateful that we could teach him that rather expensive lesson and have it stuck)