> pay as a percentage of additional salary negotiated
So many things wrong with this. Agents typically negotiate fixed-term contracts for athletes and entertainers. Agents take 10% of the entire pot, not "additional salary negotiated" whatever that means. You don't know what you would have been able to negotiate so there is no "additional" amount, only what they got you. Are you willing to give 10% of every paycheck to your agent?
Not sure why you're assuming I wanted something analogous to agents for athletes and entertainers. Those work full time and have a lot more responsibilities and provide many more services than simply doing a one time negotiation for a higher salary, none of which I'm actually looking for.
I'm looking to pay someone to negotiate a salary raise on my behalf in exchange for a cut of the additional salary negotiated. It's a one time deal, that should take a couple of emails and a couple of phone calls at most, not a full-time job. It has the potential to pay very well considering the minimum amount of effort required on the part of the "agent" as far as I can see.
Of course, I'm not denying that there could be something I'm missing that could make this arrangement infeasible (in fact, other replies have already mentioned many such factors), but I'm not sure how your post about agents for athletes and entertainers was in any way relevant to the discussion.
So many things wrong with this. Agents typically negotiate fixed-term contracts for athletes and entertainers. Agents take 10% of the entire pot, not "additional salary negotiated" whatever that means. You don't know what you would have been able to negotiate so there is no "additional" amount, only what they got you. Are you willing to give 10% of every paycheck to your agent?