The best alternatives to Adverttu are Scout, Spacewolff, and Wrapify. If these 3 options don't work for you, we've listed a few more alternatives below.
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Spacewolff is a peer-to-peer marketplace for ad space. Anyone can list any type of space for ads, and businesses looking to advertise can place ads with users directly. Think of Airbnb, but for ad space. Users are monetize all types of space, ranging from stickers on laptops and balcony signs, to radio ad space in nationally ranked programs.