I agree. I would not trust Alexa. It bases it ranking on visits from users who have Alexa installed.
However I do not understand Ranking.com either. It shows my site
www.logodesignworks.com as not having any links at all. While I have more than 200 links on my google search etc.
Anyways like some one suggested it is always best to depend on your logs and track your visitor count.
What good is it to have 150 visitors a day and about 2 sales a month?