Yes, that would be considered "unsolicited commercial email"...
You can always sell that ad space on eBay, post it as a Special Offer in that category here at the SSWT Forum, and other places like that. Obviously you want to create a page on your site (if you havent already) to appeal to potential advertisers.
Another option is to use an Ad Broker. I use Text Link Ads, but there are others that allow banner/button ads too I am sure. They are one of the more popular brokers though, so its easier to sell your space through them. http://www.text-link-ads.com
Unfortunately, Google frowns on ad sales. See:
http://www.clicknewz.com/1173/google-pagerank-debate/
You can also place affiliate links/banners in those spots to fill them, and include a link that says "advertise here" as well.