E-cigarette maker Juul Labs Inc.’s sales have been halted in China, days after the startup launched its products in the world’s biggest tobacco market.

Juul’s sleek vaporizers went on sale early last week online on both JD.com Inc. JD, +2.68% and Alibaba Group Holding Ltd.’s BABA, +0.50% Tmall, with refill pods in flavors such as mint, Virginia tobacco, mango and cream. But by the end of the week, they had been taken off both ecommerce sites. That left Juul at a loss as to why, according to people familiar with the matter.