The same dealers can demand a premium of 2,000 yuan to 3,000 yuan on the Air Jordan 1 classic series, a private dealer nicknamed Lao Lyu says.

Some shoes, such as the Yeezy, made by Adidas, have sold for almost 10,000 yuan in China, the official retail price being a fifth of that.

Lyu suggests anyone buying shoes use their common sense when they find what they want, because some are exorbitantly priced.

Xu sees buying shoes as one of life's pleasures rather than as a money spinner. He gets his information about shoes mainly from official channels, the media or shoe dealers, he says.

He has no plans to sell the shoes he owns.

"As for how much I spend, I reckon anyone can spend a bit of disposable income on their hobby without it interfering with their life."

The most he has spent on a pair of shoes is 5,000 yuan, he says.

"On average, the shoes I have bought have been marked up by 500 yuan to 1,000 yuan from the original price."

Chen Chaodi from Shanghai started to sell sports shoes at the end of 2012 when he was studying in Japan.

He sensed the rising market when he found many people in Japan and his friends at home talking about wanting to buy Air Jordan, made popular partly by the American rapper Kanye West.

So he began to buy popular sports shoes in Japan and then sold them to China and was able to sell 40 to 50 limited edition pairs of shoes a week, and he usually marks up the price 30 percent.

He continued the business on the side to earn a bit of cash after he returned to China in 2014. Now he can sell between 20 and 30 pairs a day, he says.

With years of sales experience he has built up purchase channels of his own, and most of his customers are men.

"Those born in the 1990s want to buy the shoes their idols are wearing, and those born in the 80s are buying them to relive memories," Chen says.

E-commerce has made buying shoes much easier, Xu says.

"I used to have to go to small shoe shops or abroad to get the shoes I wanted, but these days they are just one click away from my WeChat friends' circle."

Xu has neatly sorted out all the shoes in his living room, so he will have more space for future purchases.

"Every time I put on a new pair of shoes, I'll visit old friends," he says, only half jokingly.