Does anyone have any words of wisdom for authors who are considering submitting their manuscripts to an ebook site without using a literary agent or publisher?  I've been curioius about ebook sites such as the following which actually accept material from unpublished authors:

http://www.ebookmall-publishing.com/ebook-sales/index.htm

The following site actually seems to prefer submissions from unpublished authors:

http://www.panmacmillan.com/imprints/macmillan%20New%20Writing/

I wonder about using this avenue since one author of a children's book, which was published in the tradition method, told me that no real publisher will take your book if it has already been released as an ebook.

Any information would be appreciated.  Thanks in advance.