It doesn't get more traditional than a German Christmas market. For centuries, merrymakers have sipped warm mulled wine as they stroll around town squares, wandering past rows of decked out wooden cottages selling Christmas goods.

Snow globes, ornaments and gingerbread cookies are among the most popular items.

But down a quiet street in Bonn a new tradition is emerging.  

"Buying second-hand has become a trend," said Ulla Jansen, owner of a Bonn second-hand shop. "I have at least 1,000 clients who bring me their used stuff to sell and at least the same amount of people who want to buy them.”

Jansen says handbags and jewelry have been flying off the shelves, many bound for under the Christmas tree. It's a trend she picked up on a few years ago when a new wave of clients started strolling through her shop. 

"Young people, who used to be ashamed of buying second-hand stuff, are now coming to shop in thrift stores. They say they do not buy new stuff to give as gifts, they buy second hand," said Jansen.

Finja Juras says second-hand gifts are becoming popular. 

"A lot of people do it. My friend, for example, she bought me something from a thrift store and I really liked it. I think it's a good way to gift someone in a very nice and personal way," she said.

Once taboo, second-hand gift-giving has become not just acceptable, but a sought after way to fight climate change, especially among millennials and Gen Z who are increasingly sensitive to the impact their consumption has on the environment. 

"I do it for environmental reasons," said university student Clara Uhe. "I think there are so many clothes and you don't have to buy a new one when there are old ones too."

The UN Environmental Program says the fashion industry produces 10 percent of global carbon emissions and that textile dyeing is the second largest polluter of water globally. The industry's track record on water consumption is not much better. It can take around 7,600 liters of water to make a typical pair of jeans. 

All signs point towards continued growth in the resale market. In Germany, the sector is currently valued at around $1 billion. And in the US, the market is set to more than double over the next five years, from $24 billion to $51 billion, according to online resale platform ThredUP and retail analytics firm GlobalData.