Considering there are dozens of bigger and smaller publishers that's pain in the ass from buyer's perspective. Do I have to check publisher's site every day to check if site is still authorised seller? And what about sites that are both resellers and marketplaces?
yeah peeps will stop buying keys and start pirating the shit out of it again.
Not at all. I just stick with what is reasonably priced and I know are trustworthy: Steam, Humble, GMG, GoG, Amazon
If you really want more stores, I would go with resources that already check legitimate places, such as Enhanced Steam by GAFs own JShackles, which uses http://isthereanydeal.com/
They check pricing and deals via the following reputable stores, while Enhanced Steam automatically forwards that info to respective store pages for you. The stores they check are: