I would not go down the route of trying to parse websites, that will be a huge pain in the neck and impossible to maintain unless you have extensive resources (and as you mention it probably violates most site's terms of service anyway). Your best bet would be to hook into existing product databases via an API, such as Google's Search API for shopping, or maybe Amazon's API. Here's where you can start if you wanted to use Google:
https://developers.google.com/shopping-search/
Hopefully that gets you going in the right direction.
Edit: Here's a list of a lot more shopping APIs that could be good options:
http://www.programmableweb.com/apis/directory/1?apicat=Shopping