What Cannot be sold on margin?

Could you please explain in more detail what items or assets are prohibited from being traded on margin? I'm particularly interested in understanding the rationale behind these restrictions and how they differ from other types of financial instruments that are commonly traded with leverage. Is there a specific list of assets that cannot be traded on margin, or is it more of a case-by-case determination based on various factors? Also, what are the potential consequences for traders who attempt to trade these restricted assets on margin?