What I have a problem with is when people think they can include all the art they've commissioned of the character and ask several hundred dollars for it. I've seen it done before and it really bothers me.
Because the art simply isn't their's to sell; the artist retains all rights to it unless they specifically sign them over.
And I don't mean physical prints either, just digital files.
I think part of the issue is somewhat of a conundrum about digital media in general. You're commissioning the artist to make something specifically for you, it just happens to now be in a format that is infinitely distributive without any loss of tangible product. Regardless, you could likely ask the same artist to paint/draw that image for you on paper instead. You'd have to pay shipping, but otherwise they aren't likely to charge differently from their digital offerings. When you got that paper image, it would be 'yours' because you own a physical copy of it that not even the artist has anymore. But despite that, the artist still holds the copyright; you're not allowed to start making copies of that paper drawing without the artist's permission. In this sense, digital and physical media really aren't all that different in this space. While I agree somewhat that selling a bunch of a character's art shouldn't really rack up that sort of price tag, I think the commissioner is well within their rights to transfer the private-use 'ownership' of the work to other people. The artist still retains the copyright either way, there isn't really anything morally wrong with this practice since the commissioner loses their private-use of that work.
Keep in mind that when I say 'ownership' of the work, I really just mean the community's acceptance of it's use by that person; I'm not trying to actually interpret the law on that subject.