In the marketplaces people offer their products for sale. Similar to garage sales, but online. (eg. eCrater, www.1188.no).
Easy to scrape since they are usually free and do not tend to protect their data.
The usually huge online directories targeted at the general audience. (eg. Yellow Pages). They do protect their data to avoid duplication and loss of audience. See some posts on this.
They usually do not provide product online API access. It’s vital to scrape their data if you as a business owner want to do a competitor analysis. Thus you’ll be on the forefront of the market situation.
Search engines (eg. google, bing). The hardest cases, since they are very careful about repetitive use of their search results. Even though they do crawling and scraping on a regular base, the systems are not willing to be scraped in turn. My suggestion is to turn to the web services that make provision for scraping SERP (search result pages): DataFlowKit and Oxylabs.