You are trying to put the ad agency on the wrong side of the equation. Ad agencies work for the advertisers. They help the advertiser decide whether radio or TV or newspaper or cable or direct mail is useful to the advertiser. The ad agencies create the advertising image and they go BUY time or space. They don't work for the radio station or the TV station of the streamer of the newspaper and go SELL the time or space.
The radio station, the TV station, the newspaper, the Cable programmers HIRE sales reps who go SELL ads to advertising agencies who BUY the advertising in behalf of their customer, the advertiser.
I don't know if they still exist and how active they are, but there are REP firms that in past years have acted as contracted sales organizations for broadcasters. REP firms may be located in the big markets where the big agencies are located and can present a sales proposal for multiple radio stations rather than each station sending their street sales people on expensive trips to the big city.
