Maybe the fact that her saddle/back problem is 'almost sorted' might suggest that she isn't balanced enough to carry herself in canter on a circle/confined space? - just a thought :)
Maybe it would be an idea for sellers to have the horse/pony vetted first. Obviously it would be sensible to have it vetted yourself with your own vet anyway, but at least it would be difficult for the seller to hide anything major.
There should also be a grading system for horses concerning...