Much of what passes for "character designs" here is so generic that you can't claim copyright on it. You simply can't own a "design" like "red-haired girl with star tattoo". It's composed of such common elements that anyone could think of it themselves. No court would enforce your copyright. So anyone can just create their own version for free. Just like you could look at some photo on Google, become inspired by some girl's purple hair and heart tattoo and copy that for free.
I wouldn't bother if I were you. Selling characters just seems like the silliest thing. I don't mean to put you down when I say this but, why would anyone want -your- character? Why would they not just make their own?
See, that's exactly what I think when I see people set up adoptables. Granted, I see some neat designs made for adoptables, but since I draw myself, I don't quite see the point in purchasing an idea from someone
I suppose it's just coming from a person that can draw crap themselves. If I'd have to buy art, I'd just buy a commission over some design someone did on the fly and thought "I'm never using it again... let's make someone buy it" I just get too attached to my character creations, even if it's a one time affair.
So what? Kill her. Let her fade away, or better yet, change her. She is not a physical being she can be whatever you want her to be. Making her disappear isn't wasting anything. If anything is lost, it is simply some time and thought you put into making her. And since obviously she hasn't worked out for you, through this ordeal you've learned what you liked/disliked about that character and pitfalls to avoid when moving on to other creations.
Offering to sell someone nothing is absolutely asenine.