Why would another person / company besides yourself want to provide giveaways? Would they get marketing lists or opportunities to reach out to users (in that case you need to focus on sales to advertising departments of companies)? And/or would they get a share of the ad revenue (in which case you need to prioritize investment in your IT so that you generate automatic reports that are scaleable and feed into your income statements).
The initial idea was for the latter (provide the givers a large share of the ad revenue), which with eBay type traffic might actually be worth giving something away rather than selling it.
Who is your target audience? "Target" is the key word, and honestly I believe that if you ended up with a comic book-specific giveaway site vs. a smorgasboard (sp?) of random giveaways, your site will in the longer term be more successful. Your audience in that case came to you because they specifically want comic books (or whatever you focus on), and not freebie seekers with a junk email address. So advertisers will want to pay for access to this unique audience to sell them movies, other comic books, magazine subscriptions, toys, etc. Now you might get some ad revenue purely based on "random" impressions, but the advertising market is going to a place where targeting is much more preferred than broad reach because it generally has a better ROI.
Since my initial post, I have scaled the idea back and with some encouragement from the SUN community, I am going to launch the grand comicbook giveaway experiment soon. I`ll post a link once the site is up and running. The initial idea is still intriguing but would be near impossible to launch as a startup without some significant capital. It will be better for me to start with a more focused theme (as you pointed out) and see how it pans out and scale up from there (assuming success). I will let the SUN community know how it goes.