Personally as both an artist and a person who owns a ton of ocs I don't see the problem with it, however it does tend to get tricky and my view on it can shift depending on the situation.
If I create a character and end up spending a lot of money on art for said character, just to later not wanting to use it anymore for whatever reason I usually tend to either resell or trade it for something I could use. I'd feel bad letting the character stay unused for eternity when there are others who really badly want the character and to give it a new home.
Same for adoptables, these are often usually allowed to be resold or rehomed (talking about deviantart, furaffinity, twitter community etc) and nobody really has an issue with it as long as you sell it for the same or lower price than you got it for (unless extra art etc).
Then there comes to IMVU and Arthaven. Here it's somewhat of a "grey" area to resell or trade art, as most of these are usually stand-alone pieces that were either already made (premades) and sold or customs that can be a part of an already pre-existing character or a character they bought previously. I guess the whole idea behind these is that they were made without the purpouse of having it as a character / oc, unlike adoptables who were made for that exact reason.
Overall I don't really see the issue with rehoming a character (either it be for money, art, another character or gifting it) that you have put a lot of money into, as when you do buy a piece you get the right to the piece (in most cases), but I can at the same time see where it would make others feel uncomfortable as artists.
Also edit (felt I had to add this in)
There are also people who create or buy characters and buy art with intent of profiting of them in the long run (like stocks kinda?) and this I'm 100% against.