I think that is a major problem! Do not go near it! Unless you see the horse in the flesh and then get it vetted you can have the healthiest looking horse in the world but it can have terrible problems. And of course you cannot see the horse move from pictures.

Riding instructor

Um... the horse is lame at the canter and you want to canter.

I'd pass, unless you're looking for a walk trot horse.

It doesn't sound fishy; she's being honest with you. Any long term lameness is major, I'd be especially concerned if it's undiagnosed.

If the horse is lame at the canter she is lame at both a walk and a trot, when they are lame, they are lame and I would not even consider trading my old smelly tennis shoes for a lame horse.

Absolutely not. There are too many excellent horses out there for you to saddle yourself with a money drain you can't enjoy and will end up resenting. You can get an excellent horse on the cheap now, because of the crappy economy. I wouldn't take a horse that had something like that wrong with it if it was the only horse available. Even if it was just to be a companion animal, it's cost way more than it was worth in vet bills. I also don't keep just companion animals. If I have two horses I can go out an ride either and expect them to do well and behave and carry a novice rider. The only time I have anything like a companion animal is if I have a horse that has provided good service for years and is now too old to ride. I let them live out their lives in peace. However, horse are like people in that they do better when they have responsibilities and get out and move, so I keep my horses working as long as possible, though the work load may be cut way back. Good luck in finding a better horse.

I wouldn't waste my time on a lame horse no matter what.