The key is to keep following up with questions until you arrive at a point when they say "and yes, then we'd buy"
See more about it here if interested: http://blog.close.io/sales-hacking-kung-fu-how-to-virtually-...
Pre-product, when customers would say "Yes, I would buy this," we asked them to sign a non-binding LOI to that effect.
Nowadays, if a prospect says they'll buy when we have feature X, we ask them to sign a modified customer contract that says they don't have to pay until we mutually agree that the feature is done.
It has saved countless developer months spent on features that were not the real reason the customer didn't want to buy. And it sometimes causes the prospect to start saying, "well, actually, the issue is..." and then you can get the real feedback.
At least with No, you get feedback and that is valuable. The only way you can get to "no" or "yes" is by pushing the customer to give you an answer and people in general are hesitant to be that pushy.