As others have stated, the commissioner with the character is only paying for a service (drawing, traditionally or digitally) from the artist and have no rights aside from those the artist gives them with the purchase of said commission. Unless the artist is claiming these characters/designs to be entirely their own then that'd be something different.

So yeah they can sell prints of others commissions, though I'd like to be asked before this is done just for the sake of knowing so incidents like a friend seeing a commission of mine up for sale somewhere and telling me someone stole my commission or something and a shit storm begins to brew without knowing the details.