I believe many people who buy used games buy mostly used games. This is what publishers think, anyway.

Pretty much this, people used to sell WOW accounts like crazy. My ex got 900 dollars for a level 60 Priest with Anathema and the collectors edition stuff (I think she mailed them the box as a bonus) which mostly amounted to a unique pet choice that others don't get (mini diablo, a zergling, or a panda, not sure about the panda). I never heard of anyone getting banned for this that refused to share uniquely identifiable account details (such as screenshots with names/servers on them).

As well, buying gold was technically bannable, but all the high end guilds did it in vanilla (you fucking had too, high end vanilla content was brutal and farming wasn't possible for a lot of classes without weekly respecs).

As far as I am concerned I usually buy games new - although I do get second hand stuff when the game I am after is not available any longer or I cannot afford to get the new copy at that moment... Still, it is interesting that I can sell a physical copy of a game to a brick and mortar store and yet I cannot do the same thing with a digital copy of the software...

This is mostly because if you buy a CD you're an owner of that disc (not the data on it)

When you sell your hard copy, the license is transferred to another person.

While in digital distribution you are not an owner of anything (even of your account), so you can't do that. Nemo plus iuris rule means if you even "sold" your games, it would never make any legal effect.

And selling an account in most cases is against the ToS and may result in ban.

This may sound odd, but I shop at a GameStop where we can return used games we don't like but can't return new games. Since most games don't have demos nowadays, I tend to buy used for my PS3 and DS (and if I really love the game, I'll usually return or trade in my used copy and buy a new one to replace it).

I suppose the only thing close to what your asking is Green Man Gaming , you can trade back your digitally purchased games for money credit of other purchases , but you do have to put up with their rather intrusive and buggy capsule system.

http://www.greenmangaming.co.uk/

And how would you prove to GOG that you didn't keep any backup copies of the games you brought after you 'sell' them to someone else ?

You could sell it to someone, sure, just uninstall, and never reinstall.

With DRM free products this is very tricky, because there is nothing at all preventing you from keeping a local copy the game while selling the "original" to someone else.

With DRM services like Steam it would make more sense because the game is locked to one account only. If only Steam offered the option to move games from an account to another, it would make it possible for people to sell Steam games they don't want to keep anymore. But of course Steam does not want to endorse 2nd hand market, thus transferring of games is not possible.

You could say that also in Steam the original buyer might keep a local copy with a crack, but then that is no different from doing the same with CD-based games. The right solution to that problem would be to make the DRM more secure, and/or obtaining and sharing of cracks illegal (which I guess they already are in most civilized countries, as cracks contain modified game files which are still IP).