When was the last time the harp was regulated? When was the last time it was serviced or regulated? Ask to see the receipts or documentation for any work done.
You can ask for these when you go to see it in person.
I had a similar situation with a pedal harp several years ago, which had only 10 strings left on it. Mine ended up being fine after restringing and being regulated, but I had done research on what to look for in a used harp, and did a complete inspection in person before agreeing to purchase.