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If it were me, I'd contact the owner directly initially. There is no harm in asking. If it gets complicated then by all means a domain broker can be a good thing ... but they tend to cost money as well.
As for value, a lot of it comes down to what the 2 letters can be used for. 2 letter .coms will cost more than the average domain because there is only a set number of them but certain combinations will be making owners a lot less than other combinations in terms of revenue.
This is particularly worthwhile being aware of if you are buying the domain with the intention of monetising it. You may pay a premium price for a domain name that turns out not to be very profitable in terms of click revenue. Leaving you out of pocket and hoping you can flip it on for more than you bought it for.
In my opinion, best thing to do is email and ask if it's for sale and take it from there.
In my opinion, best thing to do is email and ask if it's for sale
I receive numerous emails asking if a domain name is for sale ...
Because a cold email, 99% of the time, provides little to no information about the buyer, it is usually difficult to qualify them, and accordingly, I usually pay them little attention.
I prefer talking to the individual -- whether buying or selling. Looking at a six-figure transaction, I would either want to communicate by phone, or in person -- or else write a letter (email?) worthy of that value.
Domain brokers I've sold domains to have always called.
In my experience, talking by phone affords each party the opportunity of asking and answering questions in real time, and allows an answer to be tailored to both answer the question and protect proprietary information.
AZ is the short form of Arizona - thus more expensive. UI is an obvious term online - thus higher. ET = branded by Entertainment Tonight. Then other domains like 'WE.com' go for far higher than the others.
It just depends - you can say 100-150k is the base line, and it goes up from there.