Look for Legal or Terms of use at the website that you want to scrape. There are often very expresss contractual restrictions on the use of the information posted. Absent express prohibitions in the Terms of Use for the site, simply posting a link to that site from yours is generally Ok. ;)
If you do not intend for people to distribute your software for a profit, edit the license and expressly prohibit distribution for a fee exceeding the nominal cost of reproduction. You may not have a case against past users but will prevent future abuse.