Well if an agent is legit enough, you will have some sort of agreement binded by a contract, just make sure that the stipulations are to your satisfaction before signing. If you don't sign anything, then it's not legal. We all know how many shit-talkers there are out there, a contract is the first step in weeding them out.
I give her 10% of all my bookings, and in exchange I never have to think ever again.
Someone calls me and wants to book me, I put them in touch with her. She negotiates, schedules, arranges payment, gets details and all sorts. All I have to do is show up, and play records - which is why I got into this biz in the first place.
Getting back to what the thread is supposed to be about ...
A booking agent is to take care of things for you. I don't see the point of getting a booking agent if you're going to also book things yourself (as Rob mentioned). Most DJs that I've talked to about stuff like this say they will refer you to their booking agent if you want to hire them. Again, this should be written in the contract if this is to be done.