1) Find a way to market this as win-win for you and the scraped sites. If you're perceived a net benefit to all involved, you will probably succeed. If you're not then you won't (for any of a number of reasons, including legal conflicts).
2) It is immeasurably easier to get forgiveness than permission, so I would not even try for the latter. That said you should honor any predeclared restrictions like robots.txt or clear terms of service.
3) Test out traction and interest as quickly and cheaply as possible. Launch as soon as you've got something usable (don't sweat whether it is "useful", as that is not really your decision to make).
1) Find a way to market this as win-win for you and the scraped sites. If you're perceived a net benefit to all involved, you will probably succeed. If you're not then you won't (for any of a number of reasons, including legal conflicts). 2) It is immeasurably easier to get forgiveness than permission, so I would not even try for the latter. That said you should honor any predeclared restrictions like robots.txt or clear terms of service. 3) Test out traction and interest as quickly and cheaply as possible. Launch as soon as you've got something usable (don't sweat whether it is "useful", as that is not really your decision to make).