SHENZHEN—At midnight Thursday, Zhang Ke was ready. She began clicking furiously on her smartphone, trying to snare one of four pairs of Adidas striped sneakers on sale for more than half off.

She didn’t land the sneakers. But the 28-year-old Beijing resident found other ways to divest herself of 3,000 yuan ($441), nearly a third of her monthly salary, in 28 minutes, snatching up clothes from Zara, Uniqlo and other brands.

“These...