The picture on this one: http://www.rodeoclassifieds.com/image/2954/item.html
is not very good... makes her look butt high and long backed, although she is very pretty. I would ask for better pictures.
This one: http://www.rodeoclassifieds.com/image/2949/item.html
seems to be the best conformation-wise to me, at least in the picture. On the other hand, her front legs look slightly tucked under, but again, that could just be the picture. Most of these fillies appear to have very long backs, which is a weakness. http://www.rodeoclassifieds.com/image/2945/item.html
is very pleasing to the eye... actually, she seems quite correct. I like her shoulder angle, and her front legs are nice and straight.
To have correct, strong back legs, you should be able to draw a straight imaginary line from the horse's point of buttock, down to the hock, and down to the fetlock. The above filly has that line. She does have a long back, but overall, she seems to be the best conformed horse out of the five. http://www.rodeoclassifieds.com/image/2952/item.html
Does seem to have odd front legs... they look like they shoot out in front. Also, it might be the picture, but her front left tendon looks like it might almost be bowed. http://www.rodeoclassifieds.com/image/2947/item.html
is quite nice. She has the most powerful looking shoulder of all the fillies, and most importantly, she looks the most balanced. The only fault I see is that she might be slightly cow-hocked, but it could just be the picture. Her back is a medium length.
So, now that I've changed my mind several times, here is my placing:
Also, several of the fillies were hosed down just before picture-taking... makes them looks very shiny and attractive :) ! However, it tends to take the eye away from the horse's conformation and more towards the "oooh, what a pretty (shiny) girl! ;) Try to look past the pretty coat, and see what's underneath. A hosed down coat can also hide a rough coat... ie., worms, blemishes, etc.
I would ask the owner for pictures parallel to the horse's side, and of the horse dry.
Hope that helped... good luck choosing! Let us know which one you pick. Oh, and temperament matters too, so don't take the best conformed mare if she has a nasty temperament. Things have to balance out ;) I know I'm just saying common sense, but you'd be surprised at how many people overlook a nasty temperament and buy a horse because it pretty, then sell or give it away a couple months after.