As an ecommerce owner I would agree with both Clarence and Leosghost above.
If you want one of us to advertise on your site, we want to make sure that doing so will convert to sales / revenue.
So, either a limited free / low cost introductory period, or a limited conversion-only period.
If you have a site and are looking for advertisers, the best thing I could recommend is KNOW who your visitors are and what they would most likely want to purchase. Then look for ecommerce sites that are geared toward that demographic and carry those products.
There are advertisers out there that, on the other hand, are interested in having at least SOME ads that are primarily oriented toward brand management / name recognition. I am pretty sure that they would pay LESS for that type of advertising as opposed to ads that are geared for sales conversion / revenue generation.
Just my two cents...