In English, all .com dictionary words have been registered for years. Even the not-so-common words can be sold at $xxx. The value of the domains depend on the frequency of search of those words and their interest for advertisers. Usually, nouns have the greater value, especially when they design an object or service that can be purchased/sold. Verbs are also appreciated. This is less the case for adjectives (especially plural/feminine), pronoun, adverbs and declined forms of words (like in German, latin and most Slavic languages).