First off, best of luck, it's a tough industry particularly with a Brick and Mortar retail but it's also great fun.
I've worked within a Vape shop for around... 4 years now aswell as other bits and pieces within the industry so have alittle experience.
First thing I'll say is you'll never have exactly what every customer wants, it's impossible unless you have an unlimited budget and a shop the size of an aircraft hanger. You'll get customers coming in for obscure coils that you'll have never even heard of, don't feel bad about telling them you don't have them, it's gives you an opportunity to prehaps suggest a more well known device/tank which the Coils are easier to acquire or just suggest a website they may be able to pick them up, loses you a sale but does build some good will.
Sell devices that are appropriate for the user!!! I cannot stress this one enough. Pet hate of mine where a shop will sell someone an overpriced, overpowered, low nicotine device when all they want to do is quit the cigs. If someone comes in wanting a powerful device, sell them that. If they just want to stop smoking though and they are new, don't take advantage of them. If you push a device that has a higher margin but isn't suitable, they will fail on the vape and that means long term loss of sales of liquids, coils etc. Far better to get someone off the cigs and make alittle less profit on one day but gain a potential long term customer who is no longer a smoker.
Make sure you've tried every liquid you are selling. Most customers will be guided by your suggestion, if you haven't tried them, that will be tricky! We use devices we are selling and vape liquids we are selling, that way we can give an honest opinion.
99% of your customers are not hobby vapers, they won't have a clue about rebuildables, ohms law, mechanical devices etc so don't beat yourself up trying to learn about all that and not getting the basics down. Most will just be people not wanting to smoke.
I could write about another dozen or so paragraphs but those I'd say are important for now. If you want any pointers, advice or just a chat, feel free to drop me a private message.