Contract to Broker a Vehicle on eBay

I am looking into an idea mentioned in an ebook some years ago. The author described one of his friends contacting an RV dealership. He asked the dealer to select one vehicle on his lot that he was having a hard time selling. The entrepreneur offered to sell the vehicle online on behalf of the dealer - at no cost to him. I am intrigued by this idea and would like to contact dealers in my area in furtherance of just such a joint venture/broker agreement. I'd build the auction listing and do the online promotion, accepting all costs against any profit above a previously agreed upon price. The issue is writing up a contract. I have searched for some kind of contract which would be acceptable to all three parties: the Buyer, the Seller and myself, the Broker. Such an agreement would have to stipulate the vehicle not be sold out from under me during the promotion (a period of about three weeks). And it would have to stipulate the way payment is made, so that all parties are secure at the point of sale. The state of origin is Pennsylvania.