No... I'm afraid that it's not as simple as that. Selling a built model IS a legal problem. I'm afraid that Chris is quite wrong here.
Allow me to explain. By selling the model you are obtaining financial gain. You are obtaining that financial gain from the use of someone else's copyright design. While it is true that you are obtaining some of that financial gain from your own work this does not make it legal to sell that model. Unless the designer or copyright owner has given specific consent for that.
Otherwise, in the same way, A Pirate could sell a whole disk of commercial and free models claiming that he was only selling the work he had done in compiling them onto a disk.
As you can see from this.... This is not as simple as the question first appeared to be.
However, it is so unlikely that any prosecution would be brought from such a sale, that you may as well go ahead and buy them. It's really a matter for your own personal choice.
As a legal technicality, there is a problem with doing it. But nobody is going to waste time chasing down something like that, where there is little if any prospect of a financial return for doing so.
Best Wishes,
Bspline.