Klytus that and the juniper that has been growing in the same can for 8-9 years looks good on top but because root work wasn't done will take another 8 years to reduce the root mass to managable in a small pot.
Lots of room for error, which gives insight to the grower, some will give better results, so it's still buyer beware.
Vance, I'd agree it is materials worked on with the intent to be bonsai in the future, as others have suggested root work, chops, branch/trunk wiring for shape, etc. are part of the list of work that may or may not have been done with an eye towards the future tree. Price or origin are not important as far as I am concerned as to what the definition is. Is a pine seedling grown by a bonsai nursery the same as a landscape nursery grown pine, not in my book it isn't, there is time removed from the process of making it a "bonsai", how much you pay for it depends on a whole slew of things including, age, size, nursery reputation( usually decided by quality), species, location, popularity,....... Can a seed you plant, and work on for a few years qualify, my answer would be yes, as would collection, and nursery crawls, as long as it has some foundation work done to it distinguishing it from a bush you stick in the ground in your yard to make it look nice.