Many people have tried to build businesses off building "price comparison" engines that scraped major sites.
Once you start getting any sort of traffic/revenue to speak of, you will receive a cease and desist. It's happened to dozens, if not hundreds of projects. I even worked on a small project that received a C&D from Craigslist.
You know how they say "It's easier to ask forgiveness than it is to get permission"? It doesn't hold true with page scraping. Get permission, or you will be hearing from their lawyers.
If you're lucky, it'll be early on, when you've got nothing to lose. If it's late, you may lose your business and all your work overnight, with a single letter.
Getting permission shouldn't be hard. Unless you're doing something sneaky, you're likely going to drive them additional traffic. Hell, once your product takes off, sites may be begging you, or even paying you to add their data.
