Ask HN: How does the secondary equity market work?

I invested in some startups through an equity crowdfunding platform.

For some of them I'm happy of the investments while for others I'd prefer to liquidate my equities (even at slight loss) because I'm no longer interested in the project or believing in it.

I've seen that they offer a secondary equity market where you can find buyers and sell your equities.

How does it work? Are there any fees (legal, bureaucratic, fiscal) etc that I should consider?