being a corkscrew broker

Well, it has happened again.  A collector want to off their collection, and it is beyond my reach.  So, the emails have gone out, and responses are forthcoming. 
I love that I have all kinds of contacts, but how much should one charge for negotiating a deal for someone.  10%, 20%, 25%?  Or, should I simply ask for pick of the litter.  For instance, if I am helping to sell a collection of 500 pieces, and the seller happens to have a Hull Club….should I just ask for the club, and not worry about the cash?  Or, should I worry about the cash, and forget the pick of the litter.
Oh well, we will work it out.