Good Luck on the Interview.
I was a Manager for a larger Retail Shop for about 7 years (53 employees avg.) so what I say may not apply to you, going into a smaller store. But it can't hurt.
What I looked for in Sales employees was confidence. Eye contact, firm handshakes, sitting comfortably, and NOT being afraid to say you didn't know the answer to a question...BUT you would find the answer. Those are the type of people I would hire.
You're probably going into a shop with 4 or less employees I'm guessing. But the confidence rule applies. If you're not sure of yourself, how can I trust you with my store and inventory when I want a day off.
Also you may be the Einstein of Vapes. Able to wrap a coil so tight it breaks the speed of light, and produces clouds like a fog in San Francisco. If I'm the owner I appreciate that and want that. But for B&M stores the majority of sales, and I assume Profit Margin are gonna be made from the entry level stuff. So don;t be afraid to share your knowledge. But don;t blow the sale by trying to take a newbie wanting to try this stuff out to a Kayfun with a Specialized Custom Mod that only 10 were made of.
In other words burn and churn. Plan on being bored a lot of the time in your sales. But take their money. And then on their 3rd or 4th time into the store introduce them to the higher end stuff.
But on a more important note. Let your future employer know, you will not lie to make a sale. You are selling your integrity, and to me that has no price tag. Be honest, but be helpful. I personally believe that if you start this stuff you should start in the middle class stuff, or your wasting money. I had to demand that I was sold a MVP as a starter kit, while the store owner was pushing me to a $20 basic model kit. (In fact I bought 2 MVPS cuz I believe in back-ups, but thats me). But I know cheaper up front can cost you more down the road. So if asked be honest in what you would buy and why. Some people know the cheapest starting point can be a waste of money in 2 weeks.
Oh BTW Retail has 3 days off a year. Christmas, New Years Day, and Easter, so please go in knowing if you want weekends off you are not getting the job. Be willing to work when needed, and having random days off during the week was actually kind of nice. Being the only guy on a ski lift on a Wednesday for 10 chairs either way ruled. But when your friends all want to party Friday night and you have to be to work by 8:00 Saturday morning gets tough after a while.
O.K. enough rambling from an old guy. Hope I passed along some knowledge. Good luck, have fun, and be honest.