MVPs do not necessarily need to demonstrate economic viability. Some of the best ones I've seen can't be sold at all -- for example, Dropbox's MVP was a video of someone using the prototype of the product. The thing they're Viable at is that they're just good enough to start giving you actionable information about those big freaking questions about the market you have.

"Will people pay money for this? In numbers sufficient to pay for my rent? In numbers sufficient to compensate me for my risk?" are all big freaking questions, but for some startups they are logically after questions such as "Does anybody actually want a website which tracks when their baby has an excretory event?" (+)

+ : Yep.