Another thought: A harp still in good condition tends to retain its value, and as market prices always go up, not down, there may actually be little or no depreciation. A well-made harp with good tone won't go out of date like computers do, or break down and wear out, like cars do; so take a look at some of the prices being asked by makers today and judge accordingly. A Google trawl of harp sites should net you plenty of info.

Two general retail outlets that handle harps are www.harpcenter.com and www.melodystraditionalmusic.com (don't leave out the "s") but there are also a lot of individual makers. This should give you some idea of what to charge.