Qualifications
The first qualification is that you be of drinking age. For obvious reasons. People aren't going to buy something that the salesperson refuses to try, though fortunately that's no longer out of fear of being poisoned.
You'll also need a license to sell liquor wholesale in whatever state you live in. Liquor is one of those weird things that basically amounts to a legal drug, so there are tons of laws about storing and transporting it. Getting a license, which includes a background check, will take around six months.
Then you'll need to apply for a permit with the Alcohol, Tax, and Trade Bureau. You'll need one of these since you're operating above retail. Again, it's because liquor is weird, legally speaking. Remember, there was a period (1920-1933) when it was illegal according to the United States Constitution (source).
Beyond that, you'll need to be a good salesperson. That generally means being persuasive and friendly, and having a pretty face never hurt anything. You're going to need to dress fairly well and you'll need to flatter your clients. You should also have a solid knowledge of the products you're looking to sell. If a customer has a question, you must have an answer—or you just lost a sale.