glib; smooth-talking; silver-tongued
Describes someone who speaks fluently and persuasively, often with a nuance of being too smooth or insincere. Used as a na-adjective or noun phrase.
He is a smooth-talking salesman.
Be careful not to be talked into something by a glib person.
Means 'eloquent' or 'articulate' in a more neutral or positive sense, focusing on speaking ability rather than the potentially insincere nuance of 口の達者.
A set phrase combining 口 (mouth/speech) and 達者 (skillful/proficient), literally meaning 'skillful of mouth'. The exact historical origin is uncertain, but it has been used to describe eloquent or glib speakers.