Members of the platform have expressed outrage due to a modification that led to items being listed as the wrong size.
This online marketplace has updated its measurement guidelines for women's and children's garments failing to inform all users about the change.
Numerous buyers and vendors have taken to social media saying Vinted listings now show an altered size classification than originally posted as a result of the reclassification.
The company said the revised method more closely matches other platforms.
It has advised users to review their products and adjust the sizing manually when needed, causing user dissatisfaction.
Earlier, the sizes had both one number and one letter, for instance S/8 or M/10.
The updated approach includes a letter with a size spectrum for example S/UK 8-10.
With some products, the size labels have been reassigned - for example a size L was previously a 12 but now shows as UK 16-18.
A fashion influencer, who creates plus-size fashion videos, explained the modification "makes transactions more difficult".
"Not many brands say XL is a 20-22," the way the service now categorizes.
Another user, from Glasgow, explained products she uploaded on Vinted with size 14 labels were displaying on the service as larger sizing 20-22.
"Should someone sized 20-22 purchase these they will likely be quite upset," she stated.
Charlotte Lunn, located in England, stated she felt frustrated by the change as she had to manually change the sizes on "every single listing".
"For me this is particularly frustrating, because of my ongoing medical condition and platform income has been essential for me since losing part of my income," she said.
The app has more than 100 million shoppers, with 18 million British users.
It has emerged as a preferred choice versus similar marketplaces including eBay and Depop.
This isn't the first time where an unanticipated change to the app has proved unpopular: earlier, the platform withdrew an update to its shipping options following member complaints.
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