As much as luxury companies would love to stamp out the secondhand trade in their products, it’s an impossible task. All the better for fashionistas and investors, who can both benefit from this booming business.

Shoppers have splashed out $1.3 trillion on new luxury handbags, clothes, watches and jewels over the past four years alone. At least some of that stuff will find its way onto secondhand websites. While in the past, unworn luxury goods would gather dust at the back of consumers’ wardrobes, the rise of online luxury resellers like The RealReal and Vestiaire Collective has made it easy for millions of people to sell their designer goods for cash.