I don't know if you have to use a broker, but you still have to do the contracts a certain way and they still have to be sent to Disney for ROFR.
I believe that I read somewhere that some of the resale brokers, if the seller already has the buyer, will take care of things for about $500 (title, etc. work).
I would have the seller contact a broker and ask what they offer when there is already a buyer, but also contact Disney to find out what the process is in that case.
Good luck!