How to backup PostgreSQL database (a schema) where a user has access to only one schema
Looks like usage privilege on public schema has been revoked from salesuser. So, you are getting the permission denied error while dumping the data. RESTRICTING PUBLIC SCHEMA

pg_dump, in general will backup all the schemas(sales and public) of your database.
Hence, you need to explicitly mention -n sales or --schema=schema_name while taking the pg_dump and it will work fine.

You can issue command \dn+ in psql to view the schemas and access permissions.