Warranties on pedal harps are usually five years, although I have heard it’s three years on some of the smaller ones. One doesn’t usually consider warranty in the matter of resale any more than for an automobile.
If a prospective buyer thinks lack of warranty is a deal-breaker, tell him or her to buy a new harp. People don’t usually refuse to buy a harp they really like just because of lack of warranty unless they suspect a potential repair problem or they’re chronic worriers. (I’m not implying there’s anything wrong with your harp.)
It’s highly unlikely your harp would require warranty work in the first five years. You could tell an interested buyer to have the harp checked out by a qualified harp technician to put his or her mind at ease. Maybe the buyer used the warranty issue as an excuse. You never know with some people.