It really is an interesting topic & even has coursework available. Here is what I have found so far:
The NCAA defines a collegiate athletic program as a professional education institution with amateur athletics. So any marketing that would financially benefit a student-athlete (directly) is not permitted. So basically they are saying that ticket sales is about all these marketing agencies can do for athletics. In fact, the vast majority of colleges that have hired them, since 2010, have stopped using them. The average commission is 33% of revenue. It says that SUNY Buffalo hired a firm in 2010 & in 2014 they paid the firm over $900k of $2.8mil in revenues despite a 10% decline in sales.
Notre Dame has been able to maintain their firm and have had one for many years. The firm only takes a percentage of new sales revenue. This has become common and is why most schools lose their marketing firms, as they fill up the stands and there is no new revenue available for the firms to be paid a commission on. As a result, many firms are looking for other ways to generate new revenue for the schools.
This method of monitoring who purchases tickets has become the standard. For example, I purchase the same 4-season tickets each year. My father has purchased these same seats since the year before the Stambaugh opened. If I stop purchasing those tickets and someone else picks them up, the university would pay a commission to the marketing agency. Yet again most universities are finding that season tickets sales in many programs are simply existing ticket-holders upgrading their seats (Anyone that purchases mine would sit in the 10th & 11th row isle seats). As a result, if a season ticket holder switched his or her seat 3 times, the university would pay more in commission, than the seats would generate.
I also saw where Northwestern was in some trouble because they allowed a marketing firm to use sports images (photos, video, etc.) in their attempts to attract new students. In effect, the NCAA deemed this as the university hiring an athletic agency. This makes sense as student athletes are students first and may have seen these videos. There was no mention of any penalties to Northwestern.