You should choose the best format available on the most platforms, you might also consider the target for who will be reading it.
I would not use Microsoft word. If you're creating a technical manual, you should seriously consider DocBook (it's an XML format) with docbook, you have the option to convert to different formats.
If you want to stay "on the cheap" and you're technically inclined, mac (OS X) ships with a tool called 'groff' MANY published books were actually written using troff (which groff is based on) however, it is a bit antiquated and requires you to know a about troff directives. (groff/troff is my personal favorite - but I'm a bit of a geek)
The important thing is that these ebooks be available on the majority of platforms. Plain text is by far the most universal, but has no support for fonts, images or anything other than the character set.
As far as selling them, if you want to sell them on your own using paypal, you'll need to look into IPN, which is how paypal notifies your website that so-and-so's payment went through. After a successful IPN, your website should allow them to download the file.
Basically, your visitor clicks a "buy now" button, they're forwarded to paypal, paypal sends a request "behind the scenes" of who made the payment.
When the visitor returns to your website after a visit to paypal, IPN enabled software on YOUR site enables them to download the product.
I make a free support class available for anyone wishing to develop their own paypal ecommerce solutions:
http://geniegate.com/other/paypal/
It requires some programming though. It's especially important to make sure the IPN was "valid" otherwise, a person could pretend to be paypal, faking an IPN notification, bypassing the payment.
If you don't wish to write this yourself, you can install pre-written software for the job.
http://geniegate.com/listings/payvex/
(Full disclosure, I own PayVeX)
There are other tools out there for the job as well, the thing to look for is an IPN handler.