Before the company is publicly listed, the crowdfunding shareholders can only sell (and must sell) when the Board tells them to -- in an acquisition or merger, in other words.
This type of clause often exists in shareholders' agreements between founders as well, to prevent one founder from selling a substantial part of the company to a party the others might not approve of.
If these are Regulation A+ offerings, which they must be to sell to unaccreddited investors, then shares are free to trade on secondary markets after the offering closes. These companies may even list on OTC exchange. Here's a good slide deck on it: https://www.sec.gov/info/smallbus/acsec-071916-otc-zinn-reg-...
I'm not familiar with US regulations at all, and I wouldn't pretend to give any advice about that. The information I posted about the agreements used by Invesdor is only pertinent to the Nordic markets in which they operate (Finland and Sweden). Sorry if that wasn't clear, and thanks for the info!
This type of clause often exists in shareholders' agreements between founders as well, to prevent one founder from selling a substantial part of the company to a party the others might not approve of.