Hi,

She looks lovely on the pictures, a very kind face. Does she buck every time she is asked to canter? Is it on a bend or straight line and does she buck after every jump? Sorry for all the questions i'm just trying to get a good idea
Do you know much about her background and what work she has had?
Is she fine in any every other way? I'd ask them wether she has had her back, teeth and saddle checked, she may be doing it due to discomfort.
It may just be that she is very green. Have you got a instructor or anyone you could take with you? You could run your fingers down her spine to see if she shows discomfort.
It's really hard to say without knowing more about her, but her back and saddle may need checking.
I'd possibly get her vetted too.

I bet your really excited, she does look lovely, she looks like a baby.