I'm not sure that this is the case when talking about such low amounts.

Taking hints from a similar "agent with percentage", think real estate agents. Unless an agent is working with pretty high level homes, the 3% they are getting is not significantly effected by a something like 20k up or down on a $300k house. It's an extra $600, but often their time is better spent getting the deal closed as quickly as possible for as many people as possible. If that $20k for the client takes an extra few days the haggle, the agent could have been working with another client on another $300k house deal.

In both cases, the thing that throws the agent and client out of alignment is the fact that the deals come through very infrequently, thus from the agent's perspective, they need to keep clients hustled through the door in order to be able to keep their own cash flow going, and working extra hard for a minimal gain isn't going to generally effect future gains. Most engineers would just use the same agent regardless unless the agent was spectacularly bad.

(and there are way more rules for real estate agents than there would be for a hypothetical engineer's agent)

But that would require realtors to give accurate and competitive assessments of property values, distracting from their primary hustle which is getting their client to sign an exclusive contract.

Not sure if that would pan out, or that's a large enough target audience. Just a thought.

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?

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.

Everyone talking about how great an agent would be are forgetting that under the current way every agent works, they'd be writing off 10% of their paychecks in perpetuity to that agent (or at least along a defined timeline).