Most companies keep their shareholder list electronically, rather than sending paper stock certificates. A company that goes public will give their shareholder list to a transfer agent. For example, my company used the American Stock Transfer & Trust Company, and I received paperwork in the mail listing my account number with them.
To sell the stock, it must be transferred to an account at a brokerage. I provided my account number to ETrade, as well as a scanned copy of my latest statement from AST, and they took care of moving the account. It’s only a little more complicated to tell them to move some but not all of the shares in the transfer agent account.
(If you for some reason hold paper certificates, it’s a similar process— you send the certificate in to the brokerage in the mail, along with the appropriate form, and they deposit it into your account. Some brokerages may charge a fee for this.)
The IPO underwriters generally require a 180-day lockup period for existing stockholders, so pre-IPO stockholders will not be able to complete a sale until six months after the IPO.
Originally answered on Quora: https://www.quora.com/How-do-a-companys-pre-IPO-shareholders-sell-their-shares-in-a-public-market-post-IPO/answer/Mark-Gritter