Sheila asked me to be specific. I won't go back and get the exact numbers but this will be close.
By the end of Dec. 2010 I had sold exactly 850 books at BN. By a week or so later it jumped to about 1119. They lag in reporting.
In Dec. I sold like 65 books at Amazon. Yeah I know that's a crazy difference. But I was excited because I had gotten --by 1/1/11-- to 900 which was a great achievement considering my sales are predominantly in middle reader books.
As of today at BN I've sold about 850 copies (of all my books). At Amazon it's over 100 with Dream Horse leading the way.
The audiences are really really different at the 2 sites. I do nothing to market the books. I'm not aware of any way to market middle reader ebooks, or get reviews for them. I have the blog and I don't think kids come here (I hope not!) I hope that answers the question. I'm having a You've Got Mail "My head's all woozy, what did you say again and I love plastic daisies they're so cheerful" afternoon.