I view my digital comics on either my PC, my laptop, or my TV, which is connected to my PC. The TV is my favoured platform. The comic pages can be resized to span the screen's width so I can easily view the comics from a distance of 15 feet. Nice to lay back in my recliner and 'read' the backlit TV screen. I use a wireless mouse and keyboard to navigate the pages.
However, most comics, even those created specifically for computers, still appear to be stuck with the tradition of being drawn in A4/Letter/US comic size, meaning the reader has to keep scrolling up and down each page. Of course, the more practical format is to only have 1-3 panels per page (depending how much information each panel contains), then 'slideshow' through the panels.
The most common digital formats are PDF, .cbr and .cbz (the latter two are simply renamed .rar and .zip files). Any of these formats works for me, although the latter two are allied with software offering more navigation options than PDF.
I think that it's always worth a go selling digital graphic novels (and comics) through Amazon. Even if the costs are higher than going it alone with a self-authored website (Amazon taking a % of each sale), Amazon offers more exposure and a sense of greater legitimacy.