Qualifications
Can you imagine what an ad sales hiring manager must go through? It's not like he's recruiting for a computer programming position, where each candidate must have a specifically defined skill set. Ad sales reps are all over the map when it comes to qualifications.
You'll need a few non-negotiable character traits. You must be friendly, trustworthy, and able to communicate professionally in person and on paper (or computer screen). Don't even think of using street slang or text-speak on the job. And you must be, shall we say, well-groomed, and present a neat, professional appearance. Do you need to wear a suit? Not necessarily—it depends on your business environment. But you can't roll out of bed, skip a shower, ignore your yellowed, un-brushed teeth, and slide into yesterday's (or last week's) wrinkled clothing you just found on the closet floor. College was fun, but let's leave it behind us and move on, shall we?
Okay, let's assume your grooming habits pass muster. How about education and work experience? Let's face it: If you're working directly with clients, you need some degree of decorum and business world background. Many employers want a college degree even for an entry-level position, although your previous successful sales experience might be an acceptable substitute (source). If you're bilingual, that's even better. Many advertisers are tailoring ad campaigns to diverse cultural groups, and you can best serve those clients if you speak their language. Beware those who speak Parseltongue.