Not a lot of people do that. As mentioned, it's a service idea I kicked around a bit and wrote an article about that still gets me inquiries years later. 1-5% might make it worthwhile depending on how high ticket an item you are.

The work required by an agent to place a 200K engineer isn't too much different than that of a 50K engineer, other than the 200K engineer likely being more marketable.

And the difference between 1% and 5% is substantial. I don't think I'd offer agent services for 1K, but for 5K I would.