An ad agency has a big incentive to keep your user data to themselves, as that's their most valuable and competitive resource. Google and Facebook have a huge incentive to not leak information about their users; why would someone pay them if that data was available elsewhere?
DDG has every incentive to sell user data, given that they don't have their own ad network with which to target. How else would they make money off of it?
My point is that "they're not an ad agency" is not an argument that a company has some incentive to protect user privacy, and "they're an ad agency" is not an argument that a company is incentivized to sell user data.
Yeah, okay, DDG _might_ be lying about their privacy promises. They _might_ be tracking us all ways to Sunday the way Google and Facebook do. They _might_ have some seekrit, back-door ad sales department that nobody knows about and somehow they've managed to keep secret. Which, of course, is a great method for making ad sales.
What does incentivise them is that their entire marketing is based around the privacy messaging, and thus pretty much their entire userbase considers that their primary selling point. If it ever came out that they were handing over that data to whomever, they'd be gone in an instant.
