as stated before she needs weight. If done properly shouldn't be a problem at all, but I would get a vet check to see if there are any medical/health reasons why she's thin.
It looks almost as if she stands on her toes on her front feet... something else a vet check could tell you about.
While I don't like that "She's a little skittish when she first sees thing (nothing major. she looks at it and snorts.) She's beginner safe." She could just be very honest and telling any prospective buyer about this.
If it is ONLY the FIRST time, it should be no big deal the 2nd and 3rd and so on time when she goes by a new object.
Even if she doesn't spook, bringing a horse into a new environment with new smells and everything, a horse may spook.
Montana was fine at his old place and he's a little more on edge now that he's moved, but it's to be expected.
Maybe ask the tb's owner if she can explain that a little better. and see what she says.
If she's close by, I would look at her, and if you want to get her, have the sale on the condition that she's suitable for you via a vet check.