I think the distinction is that the kit has been used for it's intended purpose, to construct a model. The kit is by definition used, even if the model has done no more than stand on the workbench to be photographed.
A scratch built model is new because although its put together form component parts the purpose of these component was to produce a new item. You can sell the whole thing as new, whereas if you broke it back down to its components you'd have to class those as used if you tried to resell them.
However, a category such as kit built/customised, would be useful; car, aircraft, ship and military modellers, and people who build and collect doll's houses must have similiar issues.