Selling of adspace on a website is damn tricky, bcos as someone said
you need to have enough traffic to make it attractive enough to sell.
even if you can get the many thousands of visitors each year, you still need to get the word out about the site - so how to sell/market this, is a good start.
Traditional Ad Agencies aren't the way to go, they will want £20k and chances are will engage in the dreaded 'Brand Ad Campaigns' which is just the wrong thing really. Too expensive an option and you can't compete with companies spending £100k per campaign etc.
PR (Public Relations) is cheaper, but will still cost £3k to £7k per campaign, for a start-up business at least....... more for continued press exposure (which you'll need). You could do it yourself, but do you know what your doing, is the question. Or the time involved to do this properly...
Free is just about available eg: Free ads/links on websites, but again time to hunt down these websites......
Link exchange is doable, again time......
There are banner exchanges, but people just use them to upload their banners, and rarely stick about to view yours/others. So what everyone needs is low-cost advertising options, preferably targeted eg: £10, £20, £50 ad slots - that kind of thing.
Try:
Reasonable cost links, targeted to the right audience.
Hire Trade show stands,
Networking events,
Email/Blog marketing,
Recommend Us tool
Bookmark this website links