For most practical purposes, there is no such thing as a "secondary market" for private company shares for "moderately successful" companies like these. The shares themselves will be subject to a shareholders agreement, the boilerplate for which prohibits their unauthorized sale.
Hire an attorney (or several) for a couple of hours to read through your agreements.
The shareholders agreement, even if 'boilerplate', may only give the company the right of first refusal on the sale of shares. Even if unauthorized sales are completely disallowed, if you find an interested buyer there are still ways to craft a legal agreement where you for all practical purposes have 'sold' the shares.
But if the company isn't very successful, there may not be any investor interest, which would make the legal details pretty irrelevant.
I'd recommend getting an attorney to read over your agreements, and also try and gauge investor interest by listing your shares on one of the secondary market marketplaces.
The type of agreement I'm alluding to transfers the beneficial value of the shares, voting rights, dividends, acquisition proceeds, etc. It's synonymous with selling the shares, when you are prohibited from doing so.
Mark Zuckerberg was on the buy side of a few of these type of deals. That's a "moderately successful" company.
