eBay is training its artificial intelligence using photos harvested from a social network popular among teenage girls for sharing photographs of their outfits without their knowledge or consent.

The online marketplace uses pictures uploaded by unsuspecting users of Chictopia, a popular “style” sharing website headquartered in San Francisco.

Chictopia, which describes itself as “the world’s largest style community”, is used to vote on the best outfits and blog about style. It also offers a shopping service, similar to British retail website ASOS’ marketplace.

When contacted by The Telegraph, Chictopia's founder and chief executive said she "was not aware" that eBay was using its pictures.

Unbeknownst to users, the photos are pooled in a database that can detect items of clothing from the photos. eBay uses this to suggest categories or pricing when someone tries to sell an item.

Chictopia is popular among teenage girls and was dubbed “the next big thing” by Teen Vogue. Chictopia states that under 13s are not allowed to use the service but there is no limit in place or age verification upon creating an account. It does not alert users that their images are going to be fed to third parties like eBay in its privacy policy.

After submitting a photo the user is asked to label each item and describe the garment, which helps eBay identify what is in the picture.

The practice is legal because eBay does not host the images, just the annotations and shape of the content.

It recently emerged that IBM is schooling its facial recognition technology on photos uploaded to image sharing website Flickr without users’ consent. Amazon, which also offers a facial recognition service, has historically refused to share where it gathers images to train its algorithms on, but insists that it is above board. Google uses its Photos and Google Images service to train its facial recognition tools.

eBay uses a database called ModaNet, which adds labels to more than one million Chictopia street style photos that were curated by an eBay software engineer while they were still at university.

An eBay spokesperson said: "We share only the annotations consisting of polygons around each fashion object for the existing data set. This way, we do not have to worry about hosting images form Chictopia on our site and thus avoiding any legal issues.

"We like to treat our work as augmenting the 'PaperDoll' data set instead of Chictopia. It just so happens that PaperDoll was created by crawling Chictopia by the original authors."