Well, everyone's struggling with Socratic/Platonic "Only and Every" definitions, but in this case, Aristotle would be a better way to proceed.
In his Physics, Book II starts out, "Of things that exist, some exist by nature, some from other causes". He would say, and actually sort of does say, that a Yamadori exists "by nature", not by human artifice. Yamadori has "in itself the source of its own production".
Nursery stock and pre-bonsai exist, on the other hand, "from other causes", as products of human art/artifice, human production. He goes on to state that there are four such causes that can be used to define such a man-made object, to differentiate it from other man-made objects, and these should be able to help us differentiate, therefore, nursery stock from pre-bonsai. Let's see.
First, a man-made thing must have a material cause, a substance of which it is made: in the case of nursery stock or pre-bonsai, the material would be the same - plants, trees, etc. This cause cannot therefore be used to differentiate the two.
Second, a man-made thing must have a formal cause, a "form or the arch-type" that is being aimed at during its production, its "essense", its "genera". This is what some folks here are trying to describe in terms of the size, shape, roots trunk, etc., trying to look for Socratic/Platonic "Only and Every" style definitions to differentiate the two. It is very difficult. It CAN be used to differentiate the two, but not without a lot of murkiness and exceptions, and so is not the best way to do so.
Third, a man-made thing must have an agent of production, which in this case also yields murky results, since a nurseryman could produce either a pre-bonsai or a nursery tree.
Fourth, the teleological cause, "that for the sake of which" a thing is done. Examining this, IMO, yields the best definitions for distinguishing nursery stock from pre-bonsai, and these distinctions have been touched on above: Nursery stock are trees made for eventual use in landscaping, and pre-bonsai are trees made for eventual use in bonsai.
Hope that helps.
grouper52