Ablackboard welcomes customers at a small shop tucked away in Shanghai with the message: "Every exchanged good has its own story. When it's reused by others, the story goes on." The 15-square-meter Shanghai Swap Shop is packed with merchandize, ranging from vintage clocks to souvenirs from around the world, to plants, guitars and jewelry.

They are packed in such a jumble that you really need to take a close look, in case you miss something you like. Old tunes are played and free candies offered, making the atmosphere cozy and casual.

"Here, customers pop in without an idea of what they want and may find something they have been after for a long while," the shop's owner Chen Jie says.

Swap Shop takes a client's unwanted goods, Chen and her husband set a price, which is negotiable, and this credit value can be used to buy something else in exchange. If there is a price differential this can be made up for in cash, or the client can bring in more goods to supplement the credit at a later date.

In the latter case, Chen will note the client's name, contact details and credit in a notebook. Then, it is up to them to return before the item they want is taken by someone else.

"We usually take a picture of each item, but sometimes it's snapped up before we can do so," Chen says.

I took a new canvas shopping bag, a new plastic travel mug and a used mouse to the shop. Chen cast her expert eye over the items, noting their textures, brands and other aspects, and priced them at 12 yuan ($1.90). I exchanged them for a new guitar tuner, which I discovered among a pile of gadgets.

The tuner cost 20 yuan, much cheaper than at musical instrument stores, and I paid 8 yuan to cover the difference.

Swap Shop has a list of goods they don't accept, such as clothes and stuffed toys, because the storage space in the attic is limited. Skincare products and cosmetics are also on the list due to safety concerns.

Chen, 30, used to work in the financial industry and was used to earning a lot of money. But she quit her job soon after she visited a similar store in Beijing, in October.

"I visited the shop three times and became passionate about the business," she says.

She started her swap shop in December.

Her husband Liao Sun, a freelance designer, was responsible for the shop's dcor and promotes it on micro blogs, while Chen uses her financial acumen to set the price of goods.

"We balance our strengths," Chen says of her husband. "Unlike second-hand stores, we care about the story behind the goods, not just their value."

Chen gives the example of a used backpack from someone who cycled from Shanghai to Tibet.

"The other day, a couple came to the shop and immediately fell in love with the backpack and bought it without hesitation, although it was a bit worn.

"When I told them the story about the bag, we were all surprised to learn the man had cycled the same route several times," she recalls.

Chen says that stories like this are her greatest joy running the shop. "I feel it's like a magical destiny that ties people together."

She admits the shop is not a very profitable business. Although the exchange price is usually double the price of the goods she receives, the money she earns is limited.

"On the one hand, most of the goods are quite cheap, on the other hand, the rent of the shop is high," she explains.

Prices of the goods in the shop vary considerably. The most expensive product sold so far is a humidity sensor, costing 1,500 yuan at department stores but sold for half the price at the swap shop. Toys, however, are sometimes free to kids.

The customer who bought that humidity sensor was a young US citizen, a regular called Scott.

"Our friendship started with a Zippo," Chen says.

The second-hand lighter caught his fancy on his first visit and when asked, Chen said she wasn't sure whether it was genuine or not.

"He was quite impressed and said I was honest. Since then he started visiting every other day," Chen says.

"When a customer comes in, we seldom introduce specific products because we want them to discover what they truly like without being influenced by our opinions," she says.

Despite its unfavorable location, the shop is often full of customers during its business hours from 10 am to 9 pm every day.

In the morning, elderly or middle-aged people typically look for useful items. In the afternoon and evening, more young people gather for vintage goods or souvenirs. Some residents who live nearby visit the shop several times a week to check for new arrivals.

Customer Han Qing, 34, says: "My family will move back to my hometown in Inner Mongolia autonomous region this year. I will donate most of my spare goods to the store so they can find good new homes," she says. "Like the store owners, I believe, 'One man's trash is another man's treasure'."