We've had great success with selling extra chicks on craigslist. We've ordered 25 chicks twice now (with another coming soon). We only intended to keep about a dozen, so we put an ad on craigslist when they were about 3-4 weeks old. They sold like hotcakes! It's especially easy to sell Easter eggers, and for some reason people around here love the black australorps. This year we even coordinated with a lady who posted that she was looking for some chicks, so we ordered extras of what she wanted. We know that if something falls through with her, we should have no problem selling them to somebody else.
We've found that many people don't want to deal with the sensitivity of the chicks for the first few weeks, but they still want them young. It works great for us to raise them for about 3 weeks, then sell some. That also gives us time to sort out who's who, since we get a mix of breeds and can't always identify all of them as peeps.