Just start selling it, even though you haven't built it yet. If it's successful then your first few customers will be disappointed because you don't deliver well, but by definition if the market demand exists there will be lots of other customers you can delight in the future. If the market demand doesn't exist (or you are bad at selling, etc...) this is the fastest way to find out.
I realize that it's probably not productive to invite a whole digression here on the whole "fake it till you make it" philosophy, but I can't help but feel that it's reckless.
Sooner or later (we may already be past that point), serious buyers will just stop speaking to anyone at all who even just remotely gives off the vibe of being a bootstrapped startup, assuming they're all just hustlers who won't deliver.
