Expo pins have taken visitors, who are eager to collect souvenirs of the mega fair, by storm.

The most popular pins are the ones distributed to special visiting groups and are not for sale to the general public. These exclusive pins general come with a hefty price tag.

Some souvenir pins distributed to VIP members at the Saudi Arabia Pavilion reach a staggering 10,000 yuan ($1,475) each on Taobao.com, the largest online trading website in the China, while those distributed to staff and VIP members at the UK Pavilion are selling for a more reasonable 400 yuan each.

But for most collectors, face-to-face trading is the way to go when it comes to finding popular pins.

"I used a gold pin from the Oman Pavilion to get five pins from other pavilions," said Medeth Alqahtani, a staff member of the Qatar Pavilion. "Later, I was told I could have gotten six."

People who have been collecting pins for a long time are widely known among pin traders.

"Pin collectors approach me every two or three days," said Alqahtani, who began collecting pins in May and has collected more than 200 till now. "They always trade something new with me."

It is a good way to establish friendships with staff from other pavilions, he said.

The hobby is contagious, it seems. Even volunteers and security guards have started collecting pins.

Zhu Lingjun, a volunteer from the Shanghai International Studies University, said she has collected 15 pins since July 12. She received her first pin from Expo organizers and others were obtained through trading.

"It is a reminder of my involvement at the Expo," said Zhu.

Despite the attractive prices offered for exclusive pins, they're not widely and actively traded online compared to other Expo-related items, like stamped Expo passports.