Handling Obsolete Listings

I've been wondering for sometime about coming up with a better way to handle obsolete listings. When I originally set up the site, it only had the companies listings. When a listing became obsolete, the site was set up to display an index of listings when that particular page address was requested. When I started adding MLS listings, I did something similar. At first, the MLS Listings received good traffic through Google search, but it's tapered off. My suspicion is that, since all these page addresses eventually point to the same content, that Google is penalizing me for that. Likewise, I don't just want to throw out a "page not found" message for someone coming to the site. So I'm looking for alternatives. I think I might try adding the Google Canonical tag.