Hi,
I've been selling an ebook for 7 years. Paypal buy now buttons are for single item purchases and a shopping cart for multiple items.
You may want to checkout
http://www.payloadz.com/ they let you use their server with their payloadz express service. They intergrate with paypal's IPN so it's transparent (they buyer never sees a paypal screen) and optionally let you list the ebook in their store as well as on ebay.
Then there is also
http://www.clickbank.com
They process creditcard payments and return the buyer to a download page you provide. Not the best but it works.
Ebooks are best in PDF format as Adobe Acrobat Reader is free and most everyone has it or can download it.
With Adobe professional version 6 or 7 you can write the ebook in MSWord and convert it to PDF or just in Adobe direct.
createpdf.adobe.com and
www.pdfonline.com offer free online conversion to pdf format. The number/size of the files are limited and some place a watermark.