I've done this a bunch of times and there's nothing wrong with it at all. Buy low and sell high, nothing wrong with that. I'm not forcing anyone to buy stuff I'm offering, just like the stores aren't forcing me to buy what they're offering. I take an item to a store, they offer me something for it, we make that exchange, everyone agrees to it. I fail to see how anything about that is unethical since the game is legally bought and owned, and nobody is getting anything other than what they agreed to.