Do not give away your used Expo tickets to strangers, police officers warned visitors.

Like many others who had a long, tough day in the Expo Garden, Ms Yu, a 25-year-old reporter, left the site from Gate 6 as night fell. When she departed, she was approached by a group of strangers.

"Whispering, they asked if I had a used ticket to sell for 5 yuan (73 cents)," said Yu. "They already had several in hand."

"There were about 7 or 8 of them, men and women, who repeatedly asked the same question to visitors leaving the Expo," Yu said.

The following day she crossed paths with another group, who were willing to pay 5 to 10 yuan for a used ticket and occasionally offered an Expo trinket in exchange.

Like many other visitors, she tried to avoid these scalpers, commonly known as huangniu in China, who she had previously encountered at railway stations.

While railway station scalpers are able to resell tickets to travelers, she could not imagine what further use might be made of Expo tickets, especially so late in the day.

"When a ticket passes the checking machine, it is marked with an indentation of '2010 EXPO'," said a police officer, surnamed Huang, who is stationed at the Yaohua Road entrance to the garden.

"If it's checked manually, a punching machine will leave a hole in the ticket. The difference separates a used ticket from a new one.

"We have repeatedly told visitors to only purchase tickets from official merchandisers," Huang stressed. "And now we are reminding them not to sell their used tickets to strangers."

According to Huang, no fake tickets have surfaced at the Expo. So what value do the used tickets have for scalpers?

"One ticket may not count, but a collection of one month or six months would be highly valuable," he said.

After being pressed, one of the scalpers disclosed the value the tickets represent. "I believe a collection of the entire 184 days could be worth over 10,000 yuan ($1,470) in the collection market," he said.

Whether or not the scalper was telling the truth, used Expo tickets have more direct benefits.

According to the Expo's official website, ticket holders can receive a 10 percent discount at 30 tourist attractions in Shanghai throughout the Expo at places such as the Science and Technology Museum, the Wild Life Zoo and at the scenic lounge on the 100th floor of the World Financial Center.

Further discounts are also available at other tourist attractions in China, such as the Huangshan Mountain in Anhui province and the Terracotta Warriors in Shaanxi province.