One man and a $9 idea validation.

Building a single page website, linking it to a mailing list and posting about how you're 'hacking a startup for $9' on Medium and HN is not idea validation. You could get 10,000,000 people signing up and that wouldn't tell you anything about whether or not it'll be a successful business … because you're asking the wrong people.

The only people who count in a mailing list or a beta are the people who'd eventually buy your product idea. Marketing it to the startup industry, e.g. people who'd be interested in it because it's a $9 startup rather than people who have adventures and want a Flipboard clone for their pastime, then you're marketing it to people who are interested in it for the wrong reasons. Those people won't become customers. It doesn't matter how many there are on your list. All you've done is fool yourself into believing you're working on something that has a market without actually knowing whether or not that's true. You have not validated your idea. You've only tapped an easy group of people who sign up to things because they're interested in signing up to things; not because they're interested in your product specifically.

Building a mailing list or a group of beta users using a 'startup list' like Betali.st is nothing more than vanity. You must take your product to the market who'll buy it to know if it will be a success.

Build a mailing list of 1,000 people from marketing on extreme sports and adventure holiday websites, and then you'll have validated something.

(Obvious caveat: This isn't true if your product is actually for startups.)