When is the last time you sold a GOG game? (Not a gift key)
That would be the last time that I sold a Steam game. And believe me, I had games on Steam that I bought and regretted in hindsight, and would just love to get rid of. GOG is a unique case in that many of the games people buy here are games that people already knew. With Steam, Gamersgate, Origin, there is a much greater risk involved in terms of quality.
And I love the way you're trying to make people out to be hypocrites even though it's blatantly obvious that Steam is nothing more than the first 'test' case in this regard. The focus is on Steam simply because it establishes a legal precedent as the company with the largest market share. Whatever principles are held to be true for Steam also hold true to GOG, Gamersgate, GMG, Origin etc. Please don't treat people like idiots in the midst of your religious fervour.
Besides, I think it should be fairly obvious that we're not really talking about selling 3, 4, 5 dollar games. For the vast majority of people, selling such games is really not worth the hassle. There's the practicality of the matter. Why spend ages trying to flog a used licence for a measly 2-3 dollars? For that matter, why bother spending time trying to find a used licence to save a dollar when you can get a 'new' one from the original provider?
No, allowing used sales is primarily about two things. Firstly, it's about keeping out-of-print media in circulation. At it stands, when a game is pulled from circulation (and there have been plenty), there is then absolutely no way to acquire such a game legally. Secondly, it's about discouraging poor quality media with the threat of resale. Good games are less likely to be placed on the used market, meaning that more people buy new. Poor or exploitative games are sold more quickly.