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There are domains that have and don't have obvious value. For everything else in between, domainers are forever looking for methods to valuate their domains. Domain valuation is not a perfect science, so the more tools and metrics you have, the better you're able to determine your domain's worth.
External metrics are useful, because they don't rely on the domain owner's opinion, and they can tell you a lot.
Two examples of external metrics would be the number of domain inquiries or offers you get, and the type in traffic your domain receives. If you've had at least one inquiry on a domain, then you know it has value to someone. If it has value to one person, it likely has value to others. If an offer price is included with the inquiry, then you have a baseline value of the domain - it could be worth more but a floor is set. If your domain is getting type in traffic, then the keywords in the domain are known and have meaning to some people, which shows value.
A not so obvious external metric you can use is to find out how many Whois counts your domains get. If your domain gets regular or a high number of Whois checks, then at least you know other people are searching for your domain, perhaps because they wanted to register it or they are thinking of contacting you about it. If a domain didn't have value, then no one would be looking it up. Some Registrars can provide the Whois information to you (see below how to find it on Godaddy).
I usually get a Whois count report on my domains at Godaddy a few times per year. The report shows the counts for the current and previous day, week and month. If you do this on a regular basis you'll likely see trends, such as which domains usually get the most counts, or no counts at all. I just did a report today and was surprised to see higher counts on some domains than usual. At least for me these counts are high, premium .com domains probably have monthly Whois counts into the 100's or 1000's.
A few of my high count domains for the previous month were:
GeocachingGames .com - 78
PrinceRupertHomes .com - 59
Scaryville.com - 58
SuperphoneGames .com - 47
HashtagBusiness .com - 34
ScratchingGames.com - 25
Plus .cc - 24
From my own reports, general trends I see are shorter .com domains (5 letters or 5 numbers, or less), get steady and higher Whois counts. Some domains get count numbers I'm surprised at, others get no counts and that surprises me too. One of my regular top Whois counts is for Plus.cc, which is strange to me because it's a .cc domain, but this domain has had a few inquiries in the past. I only acquired it because I like the keyword.
I don't know exactly how Godaddy gets the Whois counts information, if it's from human searches and/or bot searches, but the Whois counts metric is still one tool that gives you some information on your domains value.
If you do Whois counts on your domains, what are your best monthly numbers, and what do you consider to be a 'high' monthly number?
To get your Whois count report at Godaddy, go to the 'My Account' section and launch the Domains. From there go to 'Tools' in the menu, then choose 'Exportable lists' at the bottom. Click on the 'Add new export' box to the right, select 'All my domains', then Next. In this box, check the two 'Whois Searches' boxes on the far right, then Next. Enter any name for the file then click Finish. The file will show up on the resulting page within a minute or two, then you can download it and open it.