[I am not a lawyer and in light of Wily's previous reply maybe what I'm about to say doesn't hold any legal water, but I think it makes sense; even so, it is just my opinion.]
I agree, that it seems like splitting semantic hairs or defining a
distinction with no difference, but to me, there is a very real and
important difference between someone building free models and
offering them for sale as opposed to hiring yourself out as a
builder; even if you end up building exactly the same model!
First, I think it is important to explain my understanding of what a copyright
means and how it relates to publishing. With printed kits, let's
assume that the artist retains the copyright and a publisher purchases
a license to produce the kits. The license is negotiated between the
two with the knowledge that the kits will be sold for a certain price
(or the artist will be paid a certain percentage of the price). So
when the consumer buys the kit, the artist is essentially paid for his
work and then the physical kit becomes the buyer's to do with what he
will, including reselling it.
With online electronic publishing, the "consumer" is no longer simply
a buyer of a printed work. He is assuming the role of a publisher and
so must follow the rules of the license that the artist posts as the
agreement allowing you to download and print his work. He is not
downloading a kit, per se, only permission to print that kit subject to
the terms of the license. Whether or not you have permission to then
sell that kit (either assembled or not) should be stated in the
license. If it isn't stated, then one should assume that it is not
permitted.
So, it is obvious why one cannot sell the unassembled kit if the
license doesn't allow it, but what about assembled kits? When you
offer an object, any object, whether paper, clay, metal, etc. you are
selling the object, not the work that went into it. Yes, your work is
what gives it value, that makes you put a certain price on it, but the
customer is buying the object itself. If one pot took you two hours to
make and an identical one took you only an hour (because you got
faster with practice, say), a customer will not be willing to pay you
more for the first one. When you download a model, assemble it and
offer it for sale, the customer is not going to pay you for the time
it took you to build it regardless of what the model is. The reason he
is buying it is for the model itself as well as the fact that it is
assembled. When you decide to offer that particular model for sale it
is because you know people want that particular model and it is the
original designer that makes it possible for you to offer it for sale.
However, it is entirely different if someone were to bring you an
unassembled kit and offer to pay you to assemble it. Then, it is clear
that you are being paid purely for your time, and skill to assemble
it. You can even advertise that you are available for hire to assemble
kits. The distinction is that it is clear that you are being paid for
your skill alone, regardless of the model.
So even though the end may be the same, you getting paid for
assembling a free kit, the means are what makes all the difference.