The problem is you have 500 pages of regulations that are ambiguous and subject to interpretation. Clearly the first steps they're taking in enforcement relate to underage sales. That's a straightforward issue, especially in a shop rather than an online transaction. Ask for and look at the ID and you're done.
Other aspects will be implemented over time. The next step will be the beginning of next year, when you will be required to register and supply a list of all ENDS products sold and/or made by you, i.e. a coil. If you make something, a coil, a juice, etc you are a manufacturer. That opens the door to require you to file a formal application for approval documenting essentially why your product is safe and a benefit to public health (yes, I know, why is a coil safe and a net benefit to public health

).
The biggest problem for shops will be those that mix their own juice. Then you are definitely a manufacturer. You would be required to file a PMTA for every juice mix offered at every strength. Good luck with that.
The current debate, or source of anxiety, is whether or not assembling a product for a customer to use would make you a manufacturer, and subject to all the high hurdles required. Will placing a tank on a mod or inserting a battery for a customer be regarded as manufacturing by the FDA? Answer? No one really knows how the FDA will interpret their vague and ambiguous regulations when it comes to stuff like that. They do want to differentiate a retailer from manufacturer, so if you are a shop owner, remaining a retailer is probably going to have far fewer headaches than being labeled a manufacturer.
The absolute safest thing to do? Hand a shrink wrapped item to a buyer and have them pay for it, and if they then open it and have a problem, tell them to go call/write/email the manufacturer. The reasonable thing to do? Help a customer put their kit together after they buy it. By then the transaction is complete, and they own it. You're not "manufacturing" it. The normal thing to do? Go about opening and demonstrating the product according to the manufacturer's guidelines, but do not offer or sell some "aftermarket" product to them that your shop "made". Then you're a manufacturer, not a retailer.
And any house juices? Well, there it's not so ambiguous. You will be considered a manufacturer, and for now, subject to an excessive, time consuming, expensive application process with no guarantee of approval. Maybe that will change, but probably not.