For some modelrailway enthusiast I developed a small PCB carrying an Arduino Nano. It is a rather simpel device really, all it can do is power 6 LEDs and give the user the capability of letting the LEDs run certain programs like a welder, or a fireplace. That kinda stuff.
I'll be selling this PCB alongside all components and this include a pre-programmed chinese clone of an arduino nano. Atleast that is the big plan. I read on internet that it is allowed to make and produce a shield which is compatible with an arduino. Now I have a base shield for a Nano so I believe my design falls in this category.
Is this legal? I have no desire to keep my designs and source code private. So I do not mind hosting that online along side some open source licenses.
As far as Arduino is concerned, the important part is that you respect the trademarks. The Arduino hardware designs are open source, but the usual restrictions apply to the use of the "Arduino" trademark and logos. You can learn about that here: https://support.arduino.cc/hc/en-us/sections/360003242899-Logo-and-Trademarks
You should also make sure to comply with the requirements of the licenses of any hardware designs or software you use (including modifying) in your product. It should be easy to find the license type and associated requirements (they vary from one project to another at the whims of the creator).
There may be some additional safety/EMI/etc. restrictions imposed by the regulatory bodies in the jurisdictions your product is sold in, and that is certainly a whole other can of worms.