Here is a short list of great questions to ask for a potential purchase customer...

What kind of price range or payment range are you looking for?
What part of town are you looking in?
How much is your rent or current mortgage payment(s)?
How much do you think your home is worth? ( Realtors love it when you give them this information.)
How much do you have to put down? (This is always a good one because it can present so many great opportunities for you to be a problem solver.)
Do you have any questions for me? (If you can get to this one you have done well!)
Be sure and stroke your Realtor/ Builder referral source.