EULA's are almost never legally binding. In most places it's legal to modify anything you own. You are almost always entitled to sell your own property. It's when you profit specifically from modding that it becomes a grey area, and that's typically because you're enabling piracy and copyright infringement as people have already said. Depends on local law, but selling one modded unit because you no longer want it is different than modding and flipping units as a business as well.