What is the difference between custom-built and amateur-built?
I would say no difference at all. Both kit and plans-built aircraft go by many names interchangeably. Experimental category, amateur-built, home-built, custom-built all mean pretty much the same thing. By regulation they all require the owner builder to do at least 51% of the construction work. If this basic requirement cannot be met, the aircraft may be subject to certain more restrictive FAA documentation that essentially limits the final use to display and exhibition purposes only. We have never had an incident of this type of airworthiness restriction happening in our history. I have heard of it happening elsewhere. Be careful who you deal with.