Working for a manufacturing company this is of my biggest pet peeves. Our Salesforce (and customers) insist that they _would/could_ sell (or buy) our product if only it had feature x. In my experience, most of those people aren’t really interested to sell (or buy) the product regardless of feature set.
I think this is how a lot of enterprise software is sold. Potential customer says "it's missing x", sales person says "we'll build x in two weeks", product is sold, devs have to figure out how to build x in two weeks.
Both excellent questions. Oddly the tire kicker never says "If you build X then I'll buy it .. unless my needs change or someone else releases a superior product". Requesting a purchase order casts the situation in the cold light of day.