A Media Buyer can be thought of as a trader, buying low and selling high, but with ads instead of stocks. He looks for the best ad placements and aims to monetize the traffic with the highest paying ad programs.
The more expensive the spot, the greater likelihood your ad will be seen by more people.
Remember, high traffic websites are like prime pieces of real estate: well positioned on a busy street corner with plenty of passersby.
A lot of publishers and advertisers are in fact, media buyers themselves. If by chance they’re not, they’ll sure want to collaborate with one.
In other words, the media buyer’s a middleman. And because he's a solo act, it's far easier to deal with him than a bunch of small time publishers.
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