Can you sell stocks/commodities for any price you wish (either direct or market)?
You answered your own question "whether someone buys is a different thing". You can ask any price that you want. (Or given an electronic brokerage, you can enter the highest value that the system was designed to accept.) The market (demand) will determine whether anyone will buy at the price you are asking. A better strategy if you want to make an unreasonable amount of money is to put in a buy order at an unreasonably low price and hope a glitch causes a flash crash and allows you to purchase at that price. There may be rules that unravel your purchase after the fact, but it has a better chance of succeeding than trying to sell at an unreasonably high price.