The securities sold in a Reg D offering are “restricted” under US securities law and cannot be easily resold for the first year, although they can be resold in private transactions to other accredited investors. After one year, non-affiliates of the company may sell the securities publicly without restrictions (non-affiliates are investors that are not employees or executives or founders of the company, and they own less than 10% of the company).
There are currently a limited number of trading forums for the shares of early-stage privately owned companies, whether that trading takes place in private transactions with accredited investors, or publicly to all investors. New trading forums are being developed, however. A company can choose to explore having their securities (shares) quoted or posted on these trading forums, so that non-affiliates could sell them. When a company takes these steps, there can be no assurance that management will succeed in doing so, and there can be no confidence ahead of time in how much liquidity or how viable a market might develop if they did.