The key to starting a vertical ad network is salesmanship. Are you persuasive? You'll have to persuade publishers to give you access to their sites and you'll have to persuade advertisers to pay you for that access. The first step in building a successful one-man shop will be to find an advertiser who agrees to pay you a specified amount per click for traffic that meets their criteria. With such a commitment in hand, you'll find it much easier to persuade targeted publishers to give you access to their sites in return for payments per click.

Anyone who tells you that you first need to gain access to sites "so that you have something to sell" is simply wrong. You will not succeed as a one-man shop if you attempt to go that direction without deep capitalization. Find the advertiser first. Get a commitment. Then go find traffic you can sell to your advertiser at a price that leaves a profit margin for yourself after you've paid the publisher. Other advertisers and other publishers won't be hard to find when you have a happy first advertiser and a core of happy publishers. Find the advertiser first. Good luck to you.