Ahh, yes she is posty in the back legs but this is better than sickled-hocks for jumping and her hindend needs work to build muscling, but that will come.
Front legs seem ok, though to be picky she sits back on her pasterns too much and I don't like the slope of her hooves in the front. Again, could possibly be corrected by a farrier.
Ok if you really no nothing about conformation, I hope that you have someone who does shopping with you. Online assessment can only go so far, but even the best pics are moments in time, so much comes from running hands over an animal and seeing them move.
Again this mare, she has been allowed to travel naturally, for an Arab, head up, swinging along eating up the miles, now she MAY be able to jump, she may like to jump but she is going to have to learn to carry herself differently. It is my guess that you do not yet have the skill set to make that change, and I would strongly suggest looking for something that is well trained in the discipline you want to follow.
Stop shopping for online horses. If you REALLY want a decent Arabian to buy, First, there are Arabian Breeders right here to talk to. If they don't have one for sale, I'm sure that they can refer you to someone that they trust. That's where I'd go if I was in the market.
"Corporal," Arabian, Bought in 1986, (1982-2009, RIP)