Interesting question Jerusha. Rationally speaking as a harp ages it improves – the old saw has it that “a harp sounds best just before it breaks” but with modern methods and glues I’m not sure that applies so much. I guess that would be a judgement call based mostly on the sound board condition, and the mechanics.
Then there is the collector factor – some harps are no longer being made but everyone wants those still out there. I’d almost hock my car to get my hands on a Caswell Gwydion!
Many people want a Clark; I restored one made in 1925 that I bought for $500 and then sold for $2,000; I’ve seen some that old in perfect condition sell for over $4,000 and others useful only for parts.
Definitely situations where one should look at an old harp in person.