I'll agree, Flippa is pretty popular. I follow it rather closely, so I see a lot of things that make sites sell better or worse. Most of all, you can really affect how much your auction brings in with the description you use for your site. It takes a bit to make your site stand out among all of the crappy autoblogs and such on flippa. Spend a few hours describing your site well.
There are other places, like ebay or digital point. They're OK, too.
Or, work on a private sale. Approach a competitor or customer or someone you know who may want the business.
Finally, you can drop me an email with some numbers about revenue, growth, technology, prospects, and such. I'll probably make you a fair offer.
Good luck!