When we first launched, I read a few blog post that contemplated whether or not what we were doing is legal. Not only do I firmly believe that it's legal, I think the debate is somewhat moot. We hold ourselves to a much higher standard.
As a search engine, indexing the content on other sites is a privelege not a right. Publishers should clearly see value in the relationship. If they don't, they can easily ask search engines (all or a selected few that are ill-behaved) to stay away. doing this is as simple as adding a single file to their web site -- robots.txt.
As I've stated in other posts, web publishers (including commerce sites, etc.) increasingly view search engines as playing an important role in their market ecology. To play this role, however, i think companies need to follow certain unwritten rules (or net-etiquette):
1. Summarize and point, don't aggregate
I've seen oodle referred to as a classifieds aggregator. We are not an aggregator in that we don't display listings. Our job is to point our users to listings published on other sites -- and in doing so, get users off our site as quickly as possible ( love-in not lock-in).
To this end, we've provide (attributed) summaries that contain enough information for users to qualify their interest but not enough for them to take action. To respond to a listing, users need to click-through to the full listings on the publishers site.
2. Be a search engine not a publisher (or said another way, don't compete with your content partners)
It's a fundamental conflict of interest when a company attempts to both play the role of search engine and play the role of a publisher. Why would publishers let their listings be indexed by a company building a competing business off their back? It will be interesting to see how this point plays out in the coming weeks. Rumors are ablaze with Google's entry into the classified listings business. It's also rumored to be adding classifieds listings into Froogle. Will classified publishers stand for this? I'm not sure why they would.