Secondary sales let existing shareholders take partial liquidity before a full company exit. They're common at growth stages, founders sell some shares in a Series C, employees participate in a tender offer, early seed investors sell to growth funds. The price is typically negotiated against the most recent priced round.
Secondary structures vary: direct secondary (one shareholder sells to one buyer), tender offers (the company organizes a structured sale across many shareholders), and SPVs (a fund pools several sellers' shares). Each has different governance and tax implications.