That's not unethical at all. My dream morph is a bumble bee and killer bee. Those two morphs are way out of my price range so I bought a male pastel and a female spider and female pastel. Once i finally get a bumblebee and eventually a killer bee then I will sell off the pastels and spider.

It would be unethical if you just killed them, let them go in your backyard, etc. But selling them is not wrong.