FIRST: sorry this is so damn long . . . I was only going to do a brief comment but got a bit carried away . . .
I did a quick sketch . . . in MS Paint oddly enough . . . of what I think people are talking about.
Given you have already fleshed out a lot of the detail I don't know if you want to take the time to alter it, but as it's for an auction it may be worth while.
The left sketch is what it looks like you did. The blue line represents the spine. As you can see there is a HUGE gap between the left shoulder and the left breast.
I can understand that from a perspective standpoint the closer things are the greater a gap between them as oposed to thos far away. Like looking down a city street, but with people it is RARE that there is ever such a big gap and it really throws off the pic.
My first impression was, "Where is the left shoulder?" I know it's under the hair, but it's so far behind her neck line that it looks like she is purposefully arching her back and pointing her shoulders behind her. But that doesn't seem consistent with the rest of her body language so I figure it was unintentional. And it really makes that left breast look like a mutant or something kind of like it's trying to get away from her.
The right sketch in red is more anatomically correct in that there is a direct relation as to proportion and placement between breast and shoulders because where the shoulder is has a direct effect on the breast.
The second issue is with the legs and how they connect to the body
Again the sketch on the left is yours.
The way the picture looks, her body is more or less with its side to us, BUT you have her butt behind her body . . . more importantly it's her left buttcheek where her left lef connects to the body. if you try turning your body to do what is in the picture you will find yourself needing to be double jointed in some very odd spots.
PAY A LOT OF ATTENTION TO THE BELLY BUTTON.
IF you want the chest sideways but her legs pointed away from the viewer then the belly button points away, the left butt cheek is to the LEFT of her body and more importantly to the LEFT of the RIGHT butt cheek. The way it is not the left buttcheek is to the right of the right buttcheek which makes the position impossible, even though the knees are set up perfectly for this position.
IF you want the chest sideways but the belly kind of more pointed to the viewer then the butt is in the right place but the knees are off, meaning the left leg should be shorter then the right one.
In any case I'm actually somewhat doubt I made any sense there, although hopefully you got the idea. Hard to explain without showing you.
All the detail is fantastic. You have a good grasp of clothing and hair so I think all that is fine, it's just the anatomy I think could be worked on.
Try getting an action figure, or one of those little wooden art dummies, or just grab a sports illustrated swimsuit issue or a Maxim and look at the poses the women are in . . . or better yet, have a friend pose for you.
Oh . . . as to the head. It is a bit small which makes the neck look really long, but it's more of a minor issue in my opinion. Still . . . consider that the face alone is about the sice of a persons outstretched hand plus a little for forehead and hair etc.
Well . . . I hope that was helpful and good luck with the auction.