The requirement in the US for needing listing by what's called an "NRTL" (Nationally Recognized Testing Lab) is not in the NEC directly, it varies state by state and not all states require it. Most of those that do not are in places where heavy industry is not as prevalent, because (I think) they don't want to put undue burden on farmers and such to get everything listed. What happens is that many states have their own Electrical Codes, but begin with words to the effect of, "The State Code consists of the NEC, with the following added (or deleted) provisions:..." If you are selling into a state that does not have that requirement, you are likely fine without it. But then again even in states that don't do this, individual municipalities or entities may have their own local codes that do.
Nevertheless, even if not required by any local codes, there is another argument for it, that of insurance coverage. In some industries, using unlisted electrical equipment, WHEN A LISTED ALTERNATIVE IS AVAILABLE, will be covered in a rider on the insurance policy that will either cost them extra for the coverage, or void coverage of any damages resulting from the equipment. Basically it boils down to this; if there is a loss, and it is big enough for the insurance company to send out an investigator, and that investigator pinpoints the cause as being a non-listed panel, they can refuse to cover the losses. So what you will find is that many, if not most, large industrial plants will have an internal rule, regardless of local codes, stating that anything that can have an NRTL listing, MUST have an NRTL listing. Remember, UL stands for Underwriters' Laboratories, as in INSURANCE underwriters.
The easier approach to not restrict your potential available market is to use listed assemblies. UL is the most common of the NRTLs, and many of the other alternatives simply use UL's own specifications for construction, but nothing says that you must use UL per se. There is a list maintained by OSHA of all "acceptable" NRTLs for federal government work, and most states will accept any NRTL on that list.
https://www.osha.gov/dts/otpca/nrtl/nrtllist.html
Lastly, if you are an equipment OEM that has to supply a panel on every unit you make and you are already building them yourselves, you may want to consider just getting your own shop listed, it's not that hard or expensive if you can amortize it out across a lot of panels. I did it once for someone (as a consultant) who built about 20 panels per year, it worked out for them because they were ALREADY building their own panels. If you had to create a panel shop from scratch, it might be worth it to just find a good existing panel shop to work with, they are able to amortize that cost over multiple customers and volume.
