What you just said is 100% true for the diamond industry. Their slogan "A Diamond is Forever" is created because there is very little resale value in a diamond

What you just said is 100% true for the diamond industry. Their slogan "A Diamond is Forever" is created because there is very little resale value in a diamond, so they want/need their customers to keep them. DeBeers has giant warehouses full of diamonds and trickle releases them over time.