How do I structure a contract to make sure buyer don't end-run me to the seller?

[Rob Pattison]

I have an opportunity to wholesale a 20 unit apartment complex between Long Beach and Huntington Beach, within 1/2 mile of the beach.   I met the seller's broker who agreed to give me a "marketing fee" if I come up with a buyer that closes.   I'm an investor, not a licensed agent.

My question:   Before I disclose any contact information or address to the buyer, how do I make sure I ensure I receive my agreed fee?