Zhang Ningyu, 36, an editor with the social-media platform Sina Weibo, says she sold more than 100 books to these secondhand bookstores in June when she was moving from one side of Beijing to the other.

"I'd read all the books so I didn't need them. The online secondhand bookstore was a good solution."

And of course her windfall from the old books helped her buy yet more, she says.

Zhang, who has a daughter in kindergarten, says that after buying books for herself and finding those on Duo Zhua Yu and Manyou Jing were clean, she began to buy storybooks for her daughter.

"I appreciate what they do in cleaning and disinfecting each book, which I find reassuring."

Wei says: "With what we do we aim to give each book maximum value and we hope more and more used books will come into circulation, providing more choices for people who love reading. And with low prices we want to engage the interest of more people in reading books."