This is the key point. If all you are doing is scraping prices for a personal project, questionable legality is ok (even if clear legality would be preferable). But if you are trying to build a business, ya gotta stay on the right side of the law.
I don't think it's mentioned on the wiki but I think there might be a DMCA Anti-Circumvention case too if the site you're scraping has technical measures to prevent data from being copied.