Hello,
I am writing to you as a teacher with no TESOL certificate etc but just in possession of a degree and some years of teaching experience.so i got my teaching job in china because of some teaching experience which i had.But i will strongly encourage you to get an ESL diploma like TESOL or so because that will increase your chances of securing a better ESL job especially as it is becoming increasingly difficult getting an ESL job.Though as a native speaker you can effectively teach English to non-native speakers without necessarily having some teaching experience,those certificates are in a better position to securing a job for you than being a teaching requirement because they are certificates and not teachers.But then that is the only thing that convinces the employer of your abilities.