I've been on the other side of this too often too. Sales selling things that don't exist yet, promising conflicting or impossible features and of course a ridiculous deadline.

If you're doing bespoke work, sure you wait for a customer to define what they need. But if you're working on a product, then what a single customer wants is not very relevant. You build something that you think the market wants (based on market research, gut feeling, whatever) and then try selling it. Doing it the other way around is the definition of vaporware.

On the contrary, doing it the other way ensures that when you’re done someone is willing to buy what you built. It’s not about what one customer wants defining what you build, but rather having commitment from a handful of customers that they would buy what you propose to build. There’s a huge gap between securing a commitment to buy a future product and announcing vaporware you have no intention or ability to ship.