Values are very subjective to the eye of the beholder. In the case of an instrument it would be the ear.
Also the word antiqu only applies if the item is more then 100 years old. If it is vintage- which means older but not antique the value is not as high. Make sure ot has the shuttle and bobbins and any attachments. Also someone who says not a scratch on it may be honestly right or not depending on their system of evaluation. Rust, mildew etc also can do unseen damage so the surface may look great but under side may not.