I think you would find Scribd really useful:

http://support.scribd.com/entries/96701-selling-documents-on...

Also, Amazon makes it easy for publishers to sell e-books through their Kindle store:

http://www.amazon.com/gp/help/customer/display.html/ref=hp_n...

(Once you publish on Kindle, you automatically get onto Stanza, Mobipocket and some other distribution channels too).

For clip art, I highly recommend IStock:

http://www.istockphoto.com/index.php