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.

Amazon has made it much harder to scrape their site in recent months. They now give captchas to bots that try and access their site to often by the same IP address. Unless you have a very large pool of low cost proxies, crawling millions of prices on a daily basis is challenging. I think this is what he's referring to when he says """and has lately made it very difficult to get the data in other ways."""