Are you willing to price your house competitively in order to sell it in a timely manner? 

I think the most important thing with a seller is their motivation. 

Actually, our friend Margaret Rome, Baltimore Maryland has a two question listing presentation - and she has answered your question.

If we have to ask just one question - 'Let's finish the formality. Please endorse here....' or 'Are you ready to sign the contract'? or something like  that.....

Why are you selling? 

I never really thought about "One" question.  But I guess I want to find out if they are realistic and motivated.  

Are you 100% sure you are ready to sell?

Are you 100% sure you want to use me?

Are you ready to do what it takes to sell your home?

Pure and simple: Will you accept a realistic offer?

In order to reach your goal of selling your home, are you willing to enter into a professional partnership with me ?

Are you upside down?

sign here please.

This is a thought provoking post and I use most of the answers. I will add Mr. Johnson's question to my repertoire.

How soon do you want to sell?!

I would ask them if they have a price in mind. If we are too far apart I would not take the listing.

Where are you moving?

What do you think will be the most difficult aspect of selling your home?

What do you think is the most difficult aspect of selling a home?

Jeff, you pose an interesting question. I don't think I could ever get away with asking just one question, but would have to say something about getting them to sign the listing agreement with me. Without their consent to work with me, no other question would matter.

I have enjoyed reading all the responses.

Whom do you know that is also thinking about selling, buying or refinancing a home?

Do you have any questions?

What a great question.  I really had to think back to listing presentations and I frequently ask what price they have in mind for the house.

What attracted you to buy this house?

Oh that's a tough one and I can't say just one question. But I LOVED reading everyone elses' answers -- good suggestions for reference!

Will you be realistic in setting the price.

Have you made any capital improvements since you have lived here?

As Amanda and Jared, Liane (and perhaps others) said, if the owner is sufficiently motivated to sell, then everything else (pricing, condition, etc.) will take care of itself.   So my question to the owner of an owner-occupied home (always after I tour the home with them) is: "If you sell your home, where will you go?"  The answer to that question, and perhaps follow-up questions along those lines, tells me the intensity of their motivation.  Sufficient motivation will make them receptive to my explanation about pricing, condition, access, etc.

Are you willing to let me eduate you about the current market so we can determine together what we need to do to your home as well as what we will price your home at  in order to sell it quickly and for the best price by making your home the 'shiny penny' in the price range?

What is your very favorite aspect about your home and what was the main thing that caused you the purchase your home?  I always like to find out what attracted the seller to the home, when they first purchased it, as well as, what they like best about their home.  That is usually a great selling point!

I would ask them what their life goals are associated with selling their home. Its important to try and find out the reasons why people sell a home or make financial changes so you can best help.

I ask "What have you heard about the real estate market - what do you think about it?" This questions gets them talking and you find out real fast where they are coming from and if they are realistic in their price objective.

What can you tell me about your home?  Gives the seller the oppurtunity to talk up their house and give the agent many talking points if done correctly.

Will you price your home to sell? When do you have to move?

How are you going to choose your real estate agent to list your home?

Do you have the confidence in my ability to handle the sale of your home?

This is what I ask from a Loan Officer perspective, when my past clients suggest they are thinking about selling. Assuming what your net equity will be what would you like your next house payment to be?

That question usually reveals their motivational truth before they seek a Realtor.

All good questions, but why would I ever have to ask only one?

Why do you want to sell? Finding the motivation makes our job easier.