If the person you are buying for truly has everything, it may be difficult to find an antique that they don't already own. In that case, you could try to find a unique or rare version of a piece that they already have, or you could focus on finding something that has personal significance to them.
For example, you could try to find an antique that was made in the same year or location as the person you are buying for, or something that relates to their hobbies or interests. Ultimately, the best gift for someone who has everything is something that is thoughtful and meaningful to them.