In an action by brokers for services in negotiating an exchange of real property, which was not carried out, even if plaintiffs made prima facie proof of title in the customer offered by them, defendant under his denial that the customer was able to exchange, could prove that he was not able, because he had no title to part of the real property proposed to be exchanged, and that it was incumbered by a party-wall which made it unmarketable. Mutchnick v. Davis, 114 N. Y. S. 997.