I don't know about selling them. Never been a fan of selling something that comes free with a purchase, regardless of the company and product. But hey, if people are willing to pay that's their business.
Personally, I enjoy redeeming my digital comic codes so I can build a nice cloud library for my Comixology account. I, of course, prefer paper comics but having digital back-ups is nice for when I travel, want to show someone something, or want to reread a comic without digging through my boxes.
A few weeks ago I got a Lenovo Yoga laptop/tablet (great computer) that folds into a tablet. Since then, I've been buying a TON of digital comics of books I was hesitant to buy in print, because it's actually pretty fun to read comics in tablet mode. For example, they had the first issue of the new IDW TMNT book up for free. After reading that I decided to buy the first collection, then the second, and so on.