
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.
Since the marketplace is still a bit small, a "custom" option for advertisers would probably be a good way to onboard new businesses and to capture clients that are searching for areas that don't have options yet. You could even trigger it to show up when zero or limited results are found.
Side note, design is excellent.