Look for clearly stated privacy policies on Web sites. Make sure the personal information you provide is secure.
Look for fine print and hidden requirements. Most responsible sites will have a "frequently asked questions" page.
Take a close look at what information the site discloses about its own practices. A good site will provide easy access to information on how the donation process works, whether or not the site is nonprofit, the percentage of your donation or purchase that will actually go to charity, and whether or not contributions are tax-deductible.
Make sure contact information is available on the site, in case you have any questions or problems.