A domain is worth the price the buyer wants to pay for it. For instance the domain funnyvideos.cc is appraised for $ 8,200 ~ 12,000 but it may be hard to find a buyer to pay that price. I would say domain appraisal tools only give you an idea of how much a domain should sell for by does not always apply in real life.
All appraisals give fair value for the scenario when you find the right buyer. If you can't find the right buyer (or may be the buyer is not interested that much about your domain), the value could be far less. There is no appraisal company that will guarantee domain sales for appraisal value, or anywhere near appraisal value. If a appraisal company can guarantee the domain appraisal value, they should be ready to buy that domain of yours for the appraisal value if you were not able to sell it for that price!