What you describe is not at all unusual; in several instances I have recieved such panels from the manufacturer of the equipment I'm installing.
Now, that said, listing can be an issue. UL allows panel makers to produce listed panels as "certified panel shops." This is especially important when every panel is a custom panel. UL also has the ability to certify an individual panel- you'ld have to call them for details.
If nothing else, you ought to find out which UL standards apply, and review them. If you can demonstrate that you have considered them in your design, most concerns should be eliminated.
I do find it hard to accept that a home-built panel would be cheaper than one that is produced by a factory, in the thousands. I can't help but feel there is something being left out of the description.
