To successfully negotiate with an agent you need to have built up a good rapport over the course of your property search. Whilst selling agents are acting exclusively for vendors it is human nature to want to deal with people they like and all things being equal will do so.
A positive working relationship is something a good buying agent would have been nurturing for years and can call upon this goodwill to the benefit of their clients.
It is important to gain as fuller picture as possible of all parties involved and what their drivers are. If flexibility is valued, be as flexible as possible and offer a delayed or long-stop completion. You need to prove to the agent and the vendor that you will proceed with the purchase as agreed, particularly of their onward move relies upon you doing so.