Restricting based on Owner
Following are the steps to restrict User to a document based on Owner/creator.
Step 1: Role Permission Manager
Setup > Permissions > Role Permissions Manager
Step 2: Select Document Type
Select Document Type for which you want to set user permission. After permissions are loaded for selected document, scroll to role for which you want to set restriction.
Step 3: Apply User Permission
For Role to be restricted (Sales User in this case), check "If Owner".