I think the trouble is that the word is used differently in different contexts - in some cases it simply means "old", in others (eg vintage cars) it refers to a specific date range.

It is often used to imply quality but I don't think it has ever really held that meaning. However it may carry other meanings according to context. I don't know whether Wikipedia is correct regarding guitars. With concertinas, "vintage" refers to an instrument made by the old makers such as Wheatstone, Lachenal and Jeffries, most of whom ceased production many decades ago. It doesn't necessarily imply quality but does indicate that certain methods of construction were used. Modern makers (including Wheatstone as revived by Steve Dickenson) may use these "traditional" construction methods, or may make "hybrid" instruments using accordion reeds.

I agree it is overused on ebay but is anyone fooled?