You need to prepare some go-to openers. Generic enough to be used in most social situations, but also interesting enough to get an intriguing response. In your case, since you're buying from them, you could use something like: "Wow, you have such a great taste in drinks, I'm not sure what to get. Can you help me out?"
This does a few things: it's a compliment, it shows you're in need of her expertise, and it's a great segue into deeper conversation. If you're buying a bunch of beers, you can say something like: "I'm buying a bunch of beers, care to help me choose?"
And if it's a wine or spirits, you could say: "I'm not a big wine/spirits guy, can you recommend something? How about you?"
Or if you're feeling bold, you could simply say: "You look like you know how to have fun. What's your go-to order?"
Remember, the goal of an opener isn't to land a date, it's to start a conversation. Keep it light, fun, and engaging.
After the opener, your job is to keep the conversation going. Your questions can't be too deep too soon, because you're still in the rapport-building stage. Ask her about her favorite drinks, her favorite places, what she likes to do for fun.
If she's giving you IOI's, she's probably interested, so keep the conversation going, get her number, and set up a date. Remember, it's not about the opener, it's about what happens after.