Great question, I agree that can be a bit confusing. If you want to pass multiple post_ids you would pass in multiple form data fields for post_ids[] like this curl example:
curl -i -sS -X POST "http://localhost:8080/t/15/change-owner.json"
-H "Api-Key: api-key" \
-H "Api-Username: system" \
-F "username=blake" \
-F "post_ids[]=29" \
-F "post_ids[]=30"