If I were to web crawl supermarkets' websites and retrieve info and create a comparison tool for sale would that be illegal?
In the UK the Computer Misuse Act from 1990 (section 1) states that it is an offence to access a computer where:
- the access to the data is unauthorised
- the offender knows that it is unauthorised
This may extend to web scraping because, in some circumstances, you are accessing the website intending to use the data in a way that is not authorised by the website owner.
Thus, the conservative and recommended way is to ask the owner for permission.