Hello, and welcome to the forums! It's good you want to know more instead of just selling the products. You might first want to check out posts about battery safety, since it can relate to safety of your customers. Depending on where you live, you can also ask questions about making your own juice, and maybe come up with a few "house blends" to sell along with your products. Remember, you don't need to add nicotine to a juice to enjoy it. No need to add it if you don't want it.
Also, if you sell it, I do recommend you use it on a daily basis. It helps to know the products you're selling, and the pro's and con's to them. What you'll find, is that different people like different things. Many ex smokers like a tight draw and a higher nicotine level to start off, and may gradually work their way to a lower nicotine and loose draw. This is where product knowledge comes into play. Also, building coils is a big part as well, so of course, learn to make your own coils to get the right vape for the right person.
Again, welcome to the forums.