The sites I have that have a 700k in Alexa usually are getting 2000+ visitors a day. So when I buy a domain or interested in one, I check Alexa to see if it has traffic. That along with other avenues it's a good start.
If you're shelling out 10000s of dollars for a .com solely based on the traffic to it, or backlinks then you should have wide range of tools to check your future investment.
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