We are building a website which will aggregate listings from various other websites. I would build my website to add extra search filters and other useful tools around this content that the other websites lack.
We will only display the other site's listing's header and short description. The users will be redirected to the listing on the other sites when they click on the header.
Apart from viewing the other sites listings, users can create listings on our website as well. I do not plan to give any preferential treatment to our listings over other site's listings.
We do plan to show ads but they will be shown neatly on one side (like kayak.com) and there will be no ads mixed with the listings.
I assume I can expect Cease and Desist letters from some of these sites. How should I handle them? Will having ads cause issues? What other issues can I expect apart from these 2?
EDIT: I assume robots.txt is an informal deterrent? I can see sites which disallow all content to all user-agents/crawlers but Google still crawls them.