I understand the problem well. I have myself decided to sell up in France, but the market is quiet at the moment, not helped by Brexit, so not sure how well it will go. Hopefully it will give me the time to do other things, including 3D print designing and publicity.
Getting publicity and exhibiting 3D printed models is something that needs more support. Possibly it needs someone to take that on, not just for one designer though. Gn15 is a very niche market, I remember you talking about it when you first came on here. I thought it woud also be easier selling my track system, but some seem to be happy spending hundreds,probavly more the thousands of pounds of highly detailed(yet under gauge) models, and then having a second rate layout to run them on.
When Bob and myself were exhibiting the APA box modules we thought we had got the interest of the G gauge group, as they invited us to a group exhibition, but then cancelled. O9 has had a lot of interest withing the O scale narrow gauge groups, but again is a bit limited in interest. In fact it is probably the OO9 stuff that would draw more interest, and we both know members of OO9 society.
Other problem is cost,assuming you are sticking with Shapeways, as there is not much leeway. Another reason OO9 is probably a better option. If you intend to continue with your own printing, then that is a lot of time(and money), and might be better to do up your other houses, or sell one to fund work on other(s).
I, unfortunately don't know anyone,but what is needed is one person, or company to take on the sales for many 3D print designers. It would have to be driven by high sells, and minimal markup on models. Possibly one of the times I would really support a salesman to do the job, rather than someone from the hobby. Such a person , preferably with knowledge of 3D printing in general, but not necessaily, would have several irons in the fire. When I was running shop, it was interesting that one company did use a specialist salesman who had no interest in model making to visit shops. He was also the person to ask to get tickets for Man United(not my interest), so I don't think he was living purely off selling to the model hobby.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges. http://www.rue-d-etropal.com