I have helped a friend save quite a bit of cash locating cars and parts over the years. He is a good friend and his word is good. We have a future gentleman's agreement in place and I have no doubt that his word is gold. If I was brokering Up front for a cut of the pie with an unknown client I would get a deposit with the understanding that win or loose (if the cars were all located) the deposit was forfit for my time, - and get it in writing.
There are too many "baddies" out there today - unless you know who you are dealing with you gotta watch. (Just my 2cents)
Phil