It's interesting to me that, instead of internalizing what people are saying, you argue with them. If you're that convinced that your market is big enough, fucking build it.

It's even more interesting that as soon as someone gave you an actionable way to create this using existing services (a browser extension), you jumped on it because of privacy/security.

When you're looking at preferences, the range is absolutely immense. Tagging an apartment as having "a view" does not mean that that view will be of what you want. The alternative is using a degree of ML or perhaps AI to learn what kind of view you find pleasing and tailor suggestions to you. Yet, this alternative is worse for privacy than a browser extension and building it would be obscenely expensive so you'd need some hope of going after a big market to make it worth it.

You're reading too much into my words, I think. I meant the market size of "people with specific demands" is by definition larger than the subset "people with the specific demand of x". And yes, while it can't hurt starting with a small audience, you do have to find a way to broaden your market at some point. And stating 'if only 1% of the world population uses this it's still 76 million' isn't a very good way to define market size and there isn't a VC who will take this seriously.

It's funny that you bring up AirBnB because there is a well known story about their market size that Brian Chesky likes to tell:

Brian Chesky has a funny story that he tells about their continual difficulty guessing market size.

The Airbnb founders had no idea how large it was. How do you measure the size of the market for airbed rentals at conferences and political events?

Before one particularly important VC meeting, they put together a slide that asserted the size of the market was "$200 MM per year". Nate (one of the founders) felt deeply uncomfortable with this figure, and insisted they reduce it. So, they changed it to $20 MM.

Shortly before the investor meeting, Brian and Joe (without Nate) met with (now YC partner) Sam Altman, who told them "Investors like Billions not Millions, baby, so change the 'm's ' to 'b's!"

So, Brian and Joe changed the market size to $2 Billion -- unbeknownst to Nate -- who was shocked to see the much larger market size slide for the first time in the investor meeting...

Later on, after YC, when they first met with Sequoia, one of the partners there told them sequoia had been looking at the vacation rentals market for a long time -- and did they know that the vacation rental market was a $40 billion a year industry?

You asked for an opinion if this would be a good idea for a startup. I gave you my honest insight. I'm not claiming to know everything or even to be right, but I don't have to prove you wrong by quantifying why your market size would be too small. My advice to you: if you ask for an opinion you either say 'thank you', ask for clarification, or prove my assumptions wrong. Right now it seems that all you want to hear is 'it's a great idea!'.