I see no problem taking on a horse, adding good training to it (or fattening it up to proper weight - whatever that horse actually needed), and then turning around and selling it.
In fact, I gave a horse with some issues away free to a trainer with the understanding she may sell him after he had training on him. The training would help secure his future more than anything I could do for him so it was in his best interest.
What I do have a problem with is someone buying a horse for say $500, then turning around within days or weeks and trying to sell it for double or triple what they bought it for - Especially if they lied to either the original seller (don't worry - he will have a great home with us!) or lied to the new buyer (sure, we've had him for years, or we added X training on him since we got him, or we've had him long enough to know he is 100% bomb proof and kid safe).
As long as someone is honest about the horse to all people involved - then I'm okay with it.