But consumers aren't just using these sites to get rid of excess merchandise. They're also using them as a traditional marketplace, through which they can swap out their unworn items for new duds. At The RealReal, Levesque said 80 percent of the items listed sell within three days.
In 2015, its most popular brands were high-fashion labels Chanel, Louis Vuitton, Céline, Cartier and Hermès. At thredUp, which appeals to a more typical consumer base, popular brands include Zara, Topshop, Rag & Bone, Diane von Furstenberg, Banana Republic, J. Crew, Lululemon and Nike.
Because both companies are private, neither would share details about their financials. But Marino said thredUP has processed and resold more than 14 million items since it started in 2009, and the number of new items added to the site in 2015 was double that of the prior year. He added revenues are growing at a double-digit quarter-over-quarter rate.
At The RealReal, Levesque said the company would double its revenue in 2015, and expects to do so again in 2016.
"I think over the long term we expect to see the seasonal spikes in supply to actually level out because people are getting more and more routine about the way they treat the clothes they are no longer wearing," Marino said.
"The ease of it is really making it so much simpler and convenient for people to make it a regular part of their lives."