Well nearly everybody use a different set of criteria to judge
There are some basic as the popularity, type-in traffic, length, tld.... but i will tell you the most common everybody use
Their eyes
If a name compiles ok to each domainer's brain then it's ok
However there is a trap here. Domainers are professionals or at least they know a lot for this business
So if you try to get an estimate would be only an opinion and doesn't reflect the actual price you can get from an end-buyer
Also in many forums will say that your domain is worthless just because the user that place his post... doesn't own it
So take the appraisals as a trigger to discuss the opinions of people in the industry and not a bounding price range
"How often domains sell for or above their appraisal price?"
All the time