photo.net is a famous example. OpenACS 3.2.5 has auction built into the classified ads module. I don't think there is one for 4.x yet.
We are using the classified ads module (with 3.2.5) at http://hal.bookstore.usu.edu/ . We're not using the auctions module because of political reasons.
We used to have the auctions turned on at www.sv2s.com, but it's so weak that I disabled it. It's a good idea, but unless you have all the eBay features, it's not very useful. At least in the sites I've built.