They are called "Trustees of Receivership" and the "Sales Manager" is the Presiding Judge of the ________________ District Federal Bankruptcy Court.
Agencies don't sell advertising for radio stations. They create advertising from budgets of companies, then, media buyers associated with the agencies work, usually with very visible advertising reps in all market sizes, to "place" advertising for a set commission of, usually, 15%.
An agency that makes a "buy" on a particular station fully researches the station, location, demographics, ratings, competition, etc. and allocates a portion of said budget ... not because a station doesn't "choose to have a sales staff."
It doesn't work that way. There must be communication between a radio station and an ad agency, just as if that agency were a regular client of the radio station.
The short answer to your question is post number two ... very few agencies do the bulk of radio advertising buys in small markets.
Local / agency radio buys amount to (in good times) from 30 to 40 percent of a station's revenue, and that's with all indicators working well. Other than that, as little as "0%" to 10% or slightly more make up an "agency" buy that goes to monthly station revenues. A station would, literally, die with those anemic numbers.
Local / direct makes up a good 80 to 85% of revenues on most small market stations ... maybe 70 - 30% if extraordinarily "lucky."