This horse is not limping. And not gaiting either at this point. This horse is seriously LAME. Should not be ridden at all, nor sold.
There is something really wrong here. Could be many many things, but to take on this horse if you don't have to is doing yourself a disservice.
This could be only a small issue, such as abscess, could be navicular, could be leg problem, could be shoulder issue, could be neurological too.
This can run into thousands of dollars, and the owner should be shot for not doing something about this, AND for selling a horse and lying to a prospective buyer.
Do NOT buy this horse, you will regret it. Even a PPE may not get to the root of the problem, and unless you have a qualified equine lameness specialist look at the horse, and that means X-rays and other testing, you may not get the straight of it either. Not all vets are the same.
There are too many sound horses out there to get one that is not.
Run away from this one.
Saw you passed and am glad of that.
I remember another person who was all excited about a horse she was getting, in spite of several cautions from more experience horse people as to some things we saw. She got horse anyway, and has not been able to ride him but once in couple of years now. All of the things we saw, and cautioned about, were sadly right on target. She is out price of horse, hauling across the country, and many many meds and treatments, and still has no horse to ride.
You have learned a good thing about horses.