There's nothing wrong with using premade assets if you get them legally and morally. I don't think that even counts as asset flipping. Making games is hard and time consuming, it's only natural to make your work easier when possible. Many of my own games are made with free culture assets. Heck, one of them was inspired by the assets in question, and wouldn't exist without them. I make a point of crediting the author even when the license doesn't require it, and that's that. Besides, many creators sell their assets on marketplaces like Itch.io precisely so that other people don't have to make their own. It's common, accepted practice, and mutually beneficial.
Now, I hear some people grab assets from wherever, sometimes even example games for various engines, that were never meant to be reused, and pass them off as their own. That's not nice at all, and counts as plagiarism at best if not copyright infringement. What to do isn't an easy question however, and care must be taken not to accuse someone unjustly.