Suppose your target is to promot the site in natural search ranking, and you run a business site which is selling goods/services and dont have any informative/knowledge content in them, also your business is not very big brand....in that case "link building" is only resort or do you have anything else in your arsenal.

-Thanks

It is still important to get links even if it is commercial site. But what products you are looking to sell?

I agree - get backlinks from relevant sites. This will help big time. After I'm done w/ my site - 80% goes into backlinking.

Even with lousy onpage content, plenty of good backlinks will still get you rankings. Find several ways to regularly build links and keep at it for a while. I recommend submitting some articles, submitting press releases, submitting to web directories, social bookmarking, blog commenting, forum signatures web 2.0 sites like squidoo/hubpages, etc.

Yes, link building could really help your site, just make sure that you get quality backlinks for your backlinks to have and considered with value in the first place.