Hi,
Does any of you have experience with running banner ads on a site? Is it generally possible to finance a website with banner ads?

For example a Job site: I let people post job offers for free and let other people look at the offers for free as well. Only income: Advertisement.

Other example, a Real Estate site: I let people post at no cost their homes for sale, and let other people look at the offers. Only income: Advertisement.

Is this a viable type of venture?

It used to be, and will be in the future, but the market is horrible right now I'd advise against it, for now anyway.

dont try and pay for anything with banner ads they dont work any longer click through rates as low as 0.02%

You might want to go to a place like Commission Junction (http://www.cj.com) and put up some affiliate ads if you're not getting any takers for the banner ads. You're probably not going to make a living off that, but it could be a few extra bucks until your sites get going.