The securities sold in a Reg D offering are “restricted” under US securities law and can be resold via Alternative Trading Systems (ATS) to other accredited investors but cannot be resold to the public for the first year after purchase. After one year, investors that are not insiders (Affiliates) in your 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).
With the growing presence of Alternative Trading Systems (ATS) there are an increasing number of trading forums for the Reg D shares of early-stage privately-owned companies, whether that trading takes place in private transactions with accredited investors, or publicly to all investors. When a company satisfies the listing requirements for an ATS, then it can have its securities quoted or posted on ATS platforms, so that non-affiliates can sell them. Since the whole ATS category of marketplaces is very new, liquidity will take time to build.
The Reg D shares are treated differently from the ones sold under Regulation A. The one-year holding period applies even if the company has made a Reg A+ offering. The Reg A+ will make all securities in the company tradeable after their Rule 144 holding period has passed. Of course, Rule 144 does not apply to shares purchased through the Reg A+.
Public sale lockup restrictions are reduced for people or entities who are not affiliates after a year has passed since the securities were first acquired from the company. There are exceptions to the one-year lockup in the Reg D context, see below.
Holders of Reg D securities of non-reporting companies who are not affiliates, (affiliates are a type of insider) may resell in the following ways;
At any time privately in sales under the so-called "Section 4 (1 ½) exemption", on the basis of an opinion of an attorney typically only to other accredited investors
At any time privately under Section 4(a)(7) of the Securities Act to accredited investors
At any time privately to "Qualified Institutional Buyers" under Rule 144A
At any time outside the United States in reliance on Regulation S
To the public under Rule 144, one year after the securities were issued
The exemptions for private sales above all have conditions that have to be met, and the securities remain restricted. There may also be contractual restrictions on such resales or requirements set forth in the company bylaws and state law requirements have to be complied with as well.
Officers, directors, or investors who hold more than 10% of the company's securities, might be "affiliates" and their shares will be subject to additional restrictions on resale. They'll likely need a lawyer to advise them whether these restrictions apply.
In the case of affiliates, the securities are both "restricted" and "control" and investors need to hold them a year from the date on which they got them from the company before can be resold publicly. The ways in which investors can sell publicly are the same as discussed above for non-affiliates. Again, for affiliates there are limitations on the number of shares they can sell at any one time, they'll need to sell through a broker or market maker, they'll have to file a Form 144 with the SEC and "adequate current public information" must be available about the company, which means it must be compliant with Regulation A+ ongoing reporting requirements. If investors want to resell within that year, they'll need to resell them in another private offering, probably limited to accredited or institutional investors.
Once the securities are resold publicly, they are no longer restricted. Warrants are treated the same way as all other securities sold under Reg D.