What offers have the owners had so far?
It is always worth asking the real estate broker representing the seller what the terms of their received offers have been. Likely you will get a response that there have been other offers made, but usually the details are not divulged. Regardless, knowing whether any offers have been officially submitted is quite valuable on its own. This gives you an idea of the demand for the property indicating whether you need to make a faster decision or not. Along this line of inquiry you may want to ask what the bottom line price for the property would be. This will help get negotiations under way and give you a reference point for where to begin.
A question for you the reader: Have you ever negotiated the price down below the price listed for a house you really wanted?