"We have 'dogs' and 'dog food'. Come if you are interested", read the website.

An enterprising police officer in the Jiangdu district of Yangzhou, Jiangsu province, noticed that no price was mentioned and joined the online chat group. He soon figured out that 'dogs' were guns and 'dog food meant ammunition.

Police arrested the chat group manager, a Yangzhou native named Ji, on April 25. In his office they found an air gun and 1,000 lead bullets. Li confessed that he had bought them online and intended to re-sell them.

Online sales of guns and ammunition have become a persistent problem, leading experts to call for tighter regulation of e-commerce and express delivery services.

Police in Jiangdu arrested more than 300 people and confiscated some 160 air guns and 400,000 air gun bullets last year. The munitions were made and sold in at least 17 provinces across the country, said Meng Ping, a media officer with the Jiangdu police department, on Thursday.

Police traced Ji's weapons to a shop on taobao.com that was disguised as a clothing retailer.

"The shop used express delivery services to send the goods," Meng said. The shop owner, a man named Zheng based in Shanwei, Guangdong province, was subsequently arrested.

According to police, Zheng had nearly 200 people working for him and had sold ammunition to more than 210 people. Gun barrels were made in Shanwei, while the bullets were made in Quanzhou city, Fujian province.

The ring operated three factories and three warehouses in two cities, police said. In December, they arrested seven more suspects. One of them was a man named Li, a blacksmith who made gun barrels.

"He was very contrite. He said he started making gun barrels for fun, but was lured into the business by the money," Meng said.

Online sales of illegal goods have led some in law enforcement to suggest that online platforms be required to verify who is selling what to whom.

Yi Shenghua, a lawyer with Beijing's Yingke Law Firm, said that, at a minimum, e-commerce service providers should be required to make sure their services are not used to commit a crime.

But Qiao Peilei, a spokeswoman with Alibaba Group Holding Ltd, the parent company of taobao.com, said it is almost impossible for Alibaba to oversee what its millions of registered vendors are actually selling.

"Even if we make on-the-spot investigations today, it doesn't mean they will still be doing the same thing next week," she said.

Couriers were also blamed for not checking the goods they were paid to deliver.

Express delivery companies should be required to obtain the real names of shippers and recipients so that goods can be tracked, police say.

"If we can trace any suspicious shipment of goods with the real names, it will help us identify and clear up criminal activity online," said Pan Hua, deputy director of the Jiangdu police department.