Her pasterns look just fine, IMHO. Keep in mind when told that she has inadequate muscling that she is a yearling, and has probably (and rightfully) not been worked at all. She is not a halter horse, and having said that, she looks perfectly well-muscled for her age, in my opinion.
Given another year to grow up, and a year of ground work after that, and she will start looking pretty decent, I think.
A vet check for soundness is of course a must, should you decide to pursue purchasing her. :)