artchill,
WADR, it is most certainly not unethical for a headhunter to charge the candidate for his/her professional services. It is mererly uncompetitive to do so, given that virtually all headhunters get paid by the employer. The employer is not going to pay until the position is filled, so payment for failure would have to come from the candidate. If you tried that, all your candidates would go to other headhunters.
I do not want to detract from your argument re shadchanim, which is avodas kodesh and Hashem will pay for the hishtadlus. Only your comparison to headhunters was invalid.