As has been mentioned before, I don't have the experience to wind my own motors, design the electronics to operate or transmit/receive a signal, facilities to create my own batteries, cast or even machine a propeller and shaft or the joints, so I have no choice to buy these in.
Where am I going with this - in my eyes - scratch built means that for all intents and purposes, the hull superstructure, fittings etc. are made from stock materials - just like a fabricator would obtain from a supplier, so evergreen stock and preformed timber (i.e. for planks) are all acceptable, as is glues solder and brass section, its what they are turned into that makes it a wonder.
I'm not even a kit builder - I'm a kit converter, I may have made some bits that qualify as being scratch built - the masts on soveriegn, the box on the LCMIII (and as yet I do not know anyone who has converted the airfix 1/72 into a rc boat other than me - though may soon find that this is not the case, but I believe I am the first), little changes to other boats I have, and made them RC. when the cardboard boat is ever finished, it will still have an Airfix sea king on the stern - so won't be fully scratch built, will still have propriety rc equipment and drive train.
But in my eyes - I did that preceeds any idea that 'it's not correct' by rivet counters/purists etc, it may be laughed at, but who cares other than I like it, and that I appreiciate those that have the skills and equipment to produce their own hulls and fittings, however they turn out.