In a Secondary Share Sale, an allocation of shares usually held outside of the Seedrs nominee are purchased and brought into the Seedrs nominee. Typically these shares are from early investors or employees of the company seeking an early exit, and have therefore been owned external to the nominee.
On the Secondary Market, Seedrs investors are able to buy and sell shares in Seedrs portfolio companies, all of which are already held within the Seedrs nominee. There is less administrative work required and no need for issuer approval as all the shares are legally owned by the same entity, the Seedrs nominee. For more information on how the secondary market works, see this page.