A search by AFP for "Beijing" and "sick notes service" returned 49,500 results on Chinese search engine Baidu on Thursday, with vendors providing photocopies of hospital certificates with official stamps and doctor's signatures in their "product catalogue".
Such documents have long been available in China, where corruption is frequent and fake goods of all kinds are for sale.
But the country's biggest online consumer-to-consumer platform Taobao banned searches for "World Cup" and "sick notes" after a surge in offers of the certificates in recent days, the Beijing Youth Daily reported this week.
Nonetheless sellers have kept business going on China's Twitter-like Sina Weibo and other social networking websites.
"The World Cup is coming and the huge time difference may affect Chinese football fans' watching all the games. I hereby launch the sick notes providing service to meet the demand," a user with the online handle "Guitarist playing a Ukulele" wrote on May 30.
The soon-to-be-unwell can pick from a range of illnesses, from fever and fractures to abortion and Severe Acute Respiratory Syndrome -- the infectious disease that caused hundreds of fatalities in China in 2003.
"I run the business honestly and will keep your order an absolute secret," said the user.
Sick notes are mostly sold at 20 yuan ($3.20) each on one social networking website popular among students and graduates, the Beijing Youth Daily report said.
A seller said she sold around 30 notes every day and posted pictures of piles of delivery receipts as evidence, it said.
Another vendor told the paper: "Many people buy this. It's very reliable."