I see your location is Boston so I am speculating that is where you might want to do this. I don't mean to sound negative but I just dont think it will work here.
I believe Wookie's answer regarding insurance would be correct. Vincencia and Buckley would happily write you a policy for a "dive shop." I'd also recommend you seek out training from PSI-PCI for visual cylinder inspection and O2 cleaning. This is only the beginning of the rabbit hole. You'll want some training on scuba valve / manifold rebuilds (mostly straight forward), some experience working with compressors, working/operating a fill panel, etc.
I would also recommend picking up a copies of Vance Harlow's Oxygen Hackers' Companion and Scuba Regulator Repair and Maintenance books. These resources will be super helpful for anyone operating a fill station or rebuilding valves, working with oxygen and doing cylinder inspections.
Personally while I do think there is an need for more quality fill stations I don't think the Return on Investment (ROI) is there. While Boston does have a small active community of divers it does not see the same amount of traffic that places like Florida sees. This would be even more limited when most charter boats are not running in the winter. Your prime window for most divers is May to October. Outside of these months the number of active divers drops off dramatically.
I had have often thought with the closing of Cape Ann Divers that a small fill station in Cape Ann (Gloucester/Rockport) would make a of sense but I don't see how it can be a money maker or even a profitable side business endeavor unless there is another revenue stream to make it worthwhile.
The two main examples of this business model that I know of are Amigo's in Cave Country and Fill Express (now closed) in Pompano Beach. Both of these locations in Florida have the benefit of a large amount of active divers with year round diving, including a large influx of tourist / visiting divers.
Besides the initial investment costs (compressor, multiple storage banks / cascades, fill panel, whips, nitrox stick, helium and oxygen bottle leases, booster, compressor maintenance (oil filter / changes, random repairs), you then have to pay for a retail space (if you're not running out of your garage/house), electricity costs, heating/cooling.
In addition you might have to pay and train an employee to operation said fill station unless you enjoy filling tanks for 8 hours a day in your retirement AND not going diving. Working in a dive shop can really put a damper on your diving schedule. It sounds like a good idea until you're stuck with a backlog of work on Saturday morning when you could be going diving. Then you have the liability issue (e.g. what if you sell someone bad oily gas laced with CO, bad fill, etc).
What you will notice is that lot of active divers in the Boston area (myself included) have all eventually gotten their own compressors, boosters and leases for oxygen and helium bottles after not wanting to deal with the incredible time waste of going to dive shops.
I will just say this..While I think running a commercial fill station might suck, having your own personal compressor is glorious. I'll NEVER make any money back here but the time saving benefits far outweigh the cost to me.
I'm just filling for myself, my wife, and a few very close friends that I trust but I can do this myself 24 hours a day / 7 days a week in my slippers without ever leaving my garage while drinking my coffee or beer. I don't have to worry about waiting in line, I don't have to worry about short fills, inaccurate nitrox or trimix blends. I don't have to worry about if the tanks I dropped off on Friday just sat in a corner because they were forgotten about. I don't have to sit in traffic an hour and half each way to get to the dive shop. I do my own filter changes and I have a secondary filter stack on my compressor that I keep track of. I'm currently in the process of adding an in-line CO monitor to my compressor.
TL;DR. In short I highly recommend getting your own personal compressor if the logistics and incredible time suck of going back and forth from a dive shop are getting to you. Learn everything you can about it. Perhaps you will really like it and it will be good hands-on training if you do ever open a commercial fill station.
You can get a somewhat decent but slow personal compressor for around $2500-$4000. (Alkin W31, Bauer Junior II, Coltri MCH6, just to name a few examples). If you're just filling for yourself or a couple people on occasion this will probably be fine. You can find used ones cheaper than this or of course decide you want a larger capacity compressor and spend more money.
If the thought of doing regular compressor maintenance, oil changes, filter changes and babysitting/monitoring tanks while they fill does not appeal to you then that is why dive shops exist