Some custom S.I.C. are freakin amazing, but it comes down to copyrights on certain parts if I am not mistaken. Reselling a figure is one thing. Useing raw materials to make something brand new is fine too, such as fan arts. But once you take pieces from a figure, yours or not, and use them to make something new, and then sell the figure for cash, the company get all upity because you are basically selling their figure design as your own useing parts designs they have copyrights for.
Basically you can sell you Edward Elric Revoltech as it is, make a painting or photo of it to sell, but the second you take the Revoltech joints and use them for a new figure that you intend to sell, then you're on their **** list.
Even if you produce your own S.I.C. components from scratch to make a figure, they still own the rights to that component design. It would be like Bandai releaseing figures that use exact replicas of the Figma joints (which they almost did with their Pretty Cure figures...but that is another rant for another day).
"Then what about all these custom Transformers I see around here?" If I am not mistaken Hasbro gave those guys permission to use their equipment for the creation of those toys. Therefore they are unoffical hasbro products, even if they don't get any money for it. Hasbro said "we don't give a ****!" so it is legal for them to produce and sell amazing customs.
If Bandai wasn't as protective as they are of their figure designs (which also comes form the fact that the figure sculpters are actual artists) then they would be less willing to go after these guys. The Japanese figure indusrty is about as competitive as their auto industry in my eyes.