To sell your own advertising space you should have around 3,000 daily unique website visitors before any company would be interested in purchasing your Advertising space.
Although you might find some blogger or other small business that might be willing to advertise on your website with only 1,000 daily unique website visitors.
Go to forums related to your website and add a link to your website in your signature stating that you're selling advertising space. Someone might be interested. It's worth a try anyway and you can get more website traffic that way too.