Thank you Steve and J.R. for your replies.
I believe, that for the sale of real estate, the listing agreement to sell is required to have a termination date hence, by corollary in the buyer's agreement the same requirement of a termination date applies.
To say that there is no termination date is necessary is illogical.
I think Steve is right that it is voidable because of lack of mutual consent i.e. that the agent can hold or prevent the buyer from seeking other agents.
I wonder why agents should draft this kind of agreement without termination date?
Is this the practice for buyer's agents not to put a termination date?