Please, no offence, but there are many basics that you're missing.
First, there are ebook formats that are (more or less) proprietary and are not interchangeable between ebook readers.
Second, there are ebooks that are encoded so you can't simply copy it and give it to friends. This is called DRM (stands for @%#%^& in many peoples opinion).
Third, there are many companies that will sell an ebook title in multiple formats that may, or may not have the DRM restrictions on them.
Amazon isn't going to sell epub books and Sony isn't going to sell Kindle ebooks. But there are hundreds of other options as to where you can buy a book that will work on either one of those ereaders.
One of the companies that I buy a lot of books from is
Baen books. When I buy one of their ebooks I can download it in
any or
all formats.
Ebooks that don't have DRM can
very easily be converted to a great many other formats using
Calibre.
The debacle with Amazon was a public relations nightmare that I don't think will ever happen again. Faced with a similar incident I think they (Amazon) would rather stop selling the book and pay a fine than remove the ebook from someone’s Kindle again.
The best bet for you (or anyone needing to read large type) is for you to go to stores that have ereaders for you to try out. Then lurk about MobileRead for a while until you learn of the best places to get ebooks. What I, and many others, have found is that we can find more ebooks for less money than we ever could before.
In a nut shell, get an ereader that suites you and don't worry about the rest.