Very Logical Website Honesty Checker

http://www.scamadviser.com

However, still always cross-reference with:

http://www.resellerratings.com

To date, since using the two above, I've never been scammed in hundreds of online purchases.

...and whenever possible, though somewhat vague and obviously "bureaucratic", just as a third point of reference:

http://www.bbb.org
(though, not nearly as concise and reliable as the two above)