Despite marketers’ desire to buy ads within online video, they are having a tough time expanding into the format, reports Advertising Age.

Interested buyers are reportedly saying limited inventory, specifically content created exclusively for the web, is one factor that’s holding them back. Also, the audience for web video is, to date, too fragmented to meet the needs of buyers looking for significant reach. The lack of a single model for the buying and accounting of video ads is also cited as a contributing factor.

The dollars for online video buying are expected to come from television budgets, but until the online audience expands some marketers appear unwilling to make that shift. Estimates have buyers spending just 10 percent of their TV budget on streaming video by 2010, with most of that money going toward professional content from mainstream media.

Bolt Media CEO Aaron Cohen says if independent producers are going to tap into serious advertising dollars, there needs to be effective syndication of content that can bring audience reach to niche videos.

Even so, the lack of a universal rating and measurement models continues to be a stumbling block to advertisers who need to justify their expenditures.