The broker has the right to say that they will not be involved in a transaction unless you use a certain form of contract, particularly if they feel that the contract you proposed using was flawed or even illegal in some way, and so they could be exposed to liabiltiy for facilitating a sale using it. A broker cannot be forced to do something against its professional judgment or interests. Therefore, since the had the right to refuse to move ahead unless you used a different form, they are not liable for the seller deciding to not go ahead with the transaction; people and business are not liable when they do what they have a legal right to do.