The eco-benefits of resale are among the motivating factors for buyers in the key Millennial and Gen Z demographics. Ultimately, thredUP's success will be determined by its access to goods and its ability to assure quality and expedited turnaround times for both sellers and buyers. ThredUP expects to eventually process a billion items for resale every year, up from just over a million last year. "We are confident in the long-term supply opportunity," Mr. Reinhart said.