The move is aimed at providing advertisers for the UK's biggest motoring title with more options, such as microsites plus sponsorship and cross-media opportunities. Ad network AdLink currently handles Auto Trader digital sales.
Digital agency buyers will now be served by three dedicated teams from Auto Trader, each serving an individual sector: automotive, finance and insurance, and lifestyle and fast-moving consumer goods.
Naomi Lay, head of display advertising at Auto Trader, will oversee all agency activity.
Auto Trader will sell directly to agencies on a variety of advertising across its website, ranging from integrated options to more traditional, targeted campaigns.
Jason Biffin, commercial director of e-commerce at Trader Media Group, said the company is confident the change will benefit agencies.
He added: "We have spent the past 12 months building a team and new products to suit both existing advertisers and new sectors, such as lifestyle and new car advertising."
Ahead of the move to take digital sales in-house, Auto Trader is organising road shows to agencies over January and February, as well as launching an agency-only website aimed at providing research, case studies and ad opportunities.
Despite the move in-house, AdLink will continue to manage run-of-network campaigns, allowing Auto Trader to access advertisers across its network.
Meanwhile, AdLink has lost its digital sales account with Time Out London, with the listings guide bringing the sales function of its website in-house. AdLink and Time Out are negotiating over the nature of their future relationship.
An AdLink spokeswoman said: "Taking sales in-house is a natural evolution for companies such as Auto Trader.
"We help them to grow their digital operations and once they reach a certain size, they often take digital in-house."
AdLink has picked up three new account wins, striking deals to handle digital sales for Weight Watchers, Manchester Evening News and Dating Direct.