In the southern city of Foshan, Li Tianqiang and his wife sell rice noodles and other breakfast food out of a three-wheeled truck to hungry factory workers. Over the past two years, Mr. Li, 45, has bought nearly $1,000 worth of merchandise on Pinduoduo — the equivalent of around two months’ income for him. Among his purchases: an inflatable paddle boat, a fishing bag and a cherry-red motorized car for his young daughter to drive around.

Mr. Li knows he is a little addicted. And regretted purchases? He has a few.

Some were made out of curiosity. In other cases, the items were of such lousy quality that he threw them out after they arrived. The toys he has bought for his daughter — including dolls, a violin and a keyboard — have been particularly bad, he said.

It is all so inexpensive, though, that he said he didn’t mind the occasional misfire.

“It’s nothing, really,” he said of his spending on the app.

For many years, China was a byword for shoddy goods produced at mass scale. But that is changing. Wages are rising, forcing manufacturers to compete on quality. Communist Party leaders want to nurture brands known globally for their innovations. Gadget makers such as Xiaomi and Huawei are investing heavily in design, chasing cool and cachet.

To shop on Pinduoduo, however, is to be reminded that many Chinese consumers still check prices first, and that low-end suppliers remain a big part of the country’s economy. The Pinduoduo app’s main page is a bottomless cascade of groceries, fast fashion, household sundries and electronic bric-a-brac — all carrying wildly improbable price tags.