Only Advanced Worlds and Dedicated Servers support land sales.
Therefore, if you downgrade an Advanced or Dedicated-Server World to a Starter or Standard World then:
You will no longer be able to list parcels for sale.
Any parcels that are currently listed for sale will be unlisted.
Any parcels that have been sold in the past will revert to your ownership.
About Stored Worlds
Downgrading an Advanced or Dedicated-Server World to a Stored World doesn't result in losing features immediately. If you later upgrade the Stored World back to an Advanced or Dedicated-Server World then land sales will be available again, and any parcels that had been sold in the past will remain sold. However, if you upgrade the Stored World to a Starter or Standard World then you will lose the features as described above.