2. Ask them for an intent to purchase or to reserve a spot in line or whatever.
3. Build landing pages and advertise them so that you can validate your ability to acquire customers before building anything.
4. Create a human machine that looks/feels like the real machine you are going to build later.
5. Sell your fake, semi-terrible, human powered machine that pretends to be your real system until you have enough sales to justify building the real product.
6. Build and sell your real product.
If you do #1-5 before you ever build what you would normally start by building, you'll already have experience selling and marketing your product before you ever actually build it.