I think we are talking two different things so I will try to do an Example here.
Dawg, I hope you do not mind that I use you for this example.
You have sold me banner advertisement for $39.99 for a year
You have also sold me sidebar advertisement for $19.99 for a year.
Now I would like to advertise this new site that I just acquired and I would like to advertise it on a blog that is about "on this day in history." Because I feel that the people that read that might also want to check out my blog.
Now you have $60 worth of ads sold for that blog.
Would you start that blog for me for that?
You can still sell more ads to other people. You can still put your affiliate ads on the blog and so on. Just like you do on any other blog that you have.
Now here is the big questions.
How much of your startup cost for this blog did I just cover with that $60.
Did it cover one year of hosting?
Did it cover enough to buy a domain name?
Did it cover the cost for a nice theme?
Did it cover enough for the first 20 pages of info to get the blog going?
Did it cover enough for you to hire someone to submit it to 500 directories?
Now if Dawg say yes.
What did I as the buyer get?
I just got 20 pages of relevant backlinks to my site along with 20 banners ads for a year. Plus every time a new post is added I would get more ads and links my guarantee is the first 20.
This what he was explaining to me when he said "sell it first then build it."
Now I do Not have to wait for my first $60 from AdSense before I start to break even. When the site goes online I am already going to be making money, for the cost of the site has been covered.
If I would have two people to buy ads the site would already be money ahead before it even went online.