A few months ago, I posted an article about whether or not new blogs should display ads. (My conclusion: do it, but don't overdo it, even after you've gotten big.)
The other day over at ProBlogger, Brandon Yanofsky posted a great argument on the topic:
If someone runs up to you and says, "Hey, I need your help", would you reply with, "Sorry, I'm not selling right now."?
Of course not. You'd jump right in and help.
So, the right time to start selling on your blog is:
As soon as you identify a problem and create a solution to that problem.
The very first post on a new blog could be, "I've been struggling with this problem forever, and I just figured out how to solve it! So I decided to create a blog on the subject."
Would that seem unnatural? Spammy?
Well, it could be spammy, depending on how you do it. But the point is that there's nothing inherently spammy about selling from the get go.
I'm with Brandon. If you've got something worth buying, then sell it! Sell it boldly! Sell it honestly. But boldly!