N/H means she is a carrier of the HYPP Disease, not Not Handled (good inferencing though!!)
N/N - Not a Carrier for HYPP
N/H - Carries the HYPP disease (though it may lie dormant and not be obvious)
H/H - HYPP infected horse
"Hyperkalemic periodic paralysis (HYPP) is an inherited disease of the muscle which is caused by a genetic defect. In the muscle of affected horses, a point mutation exists in the sodium channel gene and is passed on to offspring."
Source: Horse HYPP
UC Davis Veterinary Website
Isn't HYPP what Vidaloco lost her horse, Fancy to? Fancy's story
HYPP was believed to be stemmed from an AQHA Stud named "Impressive". A HYPP positive horse (H/H) and even an HYPP carrier (N/H) Will develop a BEAUTIFUL physique at a young age without much work from the handler. They always had an edge in the show ring because of this. Some N/H horses are really easy to tell apart because when they are just days old they are WAY too muscled.. like, there is no way they could have formed that much comprehenzive muscle in the first few days of life. These H/H and N/H horses kept winning in the show ring so more and more people started breeding them, then suddenly these horses started having seizures and dying.. so they found out about the disease.
AQHA and APHA (i believe) no longer recognizes and registered a H/H horse. I believe in the last few years they are trying to put a ban through to not allow old registered H/H horses, and current N/H horses to show alongside N/N horses (its literally just not fair to show a carrier with a healthy horse.. the carrier more often than not wins because of the muscle tone and whatnot). You can still register a N/H horse, but im not sure if your allowed to breed it (?) or you have to breed to a non carrier in order to have the baby be registered (?) again, that's all up to speculation.. I just heard this talked about in my local Horsemans Council. It would make sense if they did that though. People who have non carrier work so hard to put honest muscle mass on their horses, only to lose to one that is diseased.
I would not purchase this horse unless she -really- cheap. I could find a better, non diseased mare for the price of 1,700 (especially in this horse-over-populated-world).
Its a shame too, because she is REALLY beautiful, but you'll need to watch what you feed her like a hawk, and have to accept the fact that she may die very suddenly at a young age.
Google some images of H/H and other N/H horses and look at the difference between the N/N horses and some of the N/H. I would post some pictures here, but they aren't my horses to post :p don't want to step on any toes! =)