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There is a conflict of interest.
So their goal and yours are pretty much the same, making more money.
I know this from personal experience selling direct ads to customers on one of my sites because the more money I help the customer make, the more ad budget the customer will spend on my site. Likewise, if the customer isn't getting the ROI, they pull the ad campaign and I make squat. Therefore, the customer and I have the same basic goal to make the customer ad campaign as lucrative as possible and that's not a conflict of interest.