The automated valuation tools can only serve as useful in the sense of offering a rough guide to help you narrow your lists down to the real candidates. You then have to do the real valuation work yourself.
From what I see and hear, Estibot among others is using criteria which are so far out of date as to be virtually meaningless in today's terms. I'm with
@Samer in having a preference for NameWorth (there's a link to it at the bottom of each page here on NP).
The thing to bear in mind when using NW, though, is that it is designed for marketers who have a good idea of how to get the name being valued in front of the most relevant buyers within the businesses the domain is most relevant for. It is not much use at all for those who stack up large numbers of unrelated domain names and sit awaiting buyers to just, sort of, well, appear. From somewhere.
There are lots of online valuation tools out there, both free and pay to use, besides those most frequently mentioned. Used properly, without expecting miracles, as they are intended to be used, they can be a great help in sorting shortlists down to your final candidates, or for getting to the "rough idea" stage of pricing what you already have available.
Why not try some searches and test a few? See which give you the best results for your purposes? Bear in mind the "rough guide" element and always use your own judgement to overrule as necessary.
That's my twopenn'orth on the subject.
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