This is how I handle it:
"My company has different programs available to buy your house and in in order to make an educated decision on which one would best fit your needs, I need to have certain information. One of those pieces is the loan information, without it, I won't be able to help you (or make you an offer)."
If they are not willing to give you this or any other information, they do not sound motivated enough to work with anyway. Next!!
Joe