So basically if you download it, once. (or delete any previous versions). And then vow never to download it from the service again (doubt many people will hold to this realistically), then you can legally sell it.
The ease of "cheating" isn't really a criteria for whether you can resell your property or not in the US. After all, you can rip a DVD or music CD (or even a game CD/DVD) before reselling them too. The fact that one can cheat by violating the copyright holder's rights doesn't preclude you exercising your rights.
I would imagine a certain percentage of people do rip their music CDs and their movie DVDs before reselling or trading them. Certainly many rent these items and rip them. You can rip music right off of Youtube as well, with one click in many cases.
Basically it would be up to congress, in the US, to change this. Copyright is granted, so all other rights are presumed to rest with "the people", i.e. everyone as a whole. That's why fair use exists at all as a legal concept.
So yes, you can resell a digital item, there's currently at least one mp3 reselling service active in the US (yes, there's a lawsuit over their existence, last I heard they were weathering it okay).
Whether it's legal to sell the copy is irrelevant if this option has already undermined the existence of a market supporting it. Unless a market evolves around trading digital games, like from GOG that *is* something that has been lost in the transition to GOG.
As far as the US case law is concerned that is beside the point. GOG (or any other digital service) may be within its rights to terminate their service to you, but if they "sold" you an item (the button is labeled "Buy" after all) then it will be held to be a sale in the US.
I don't care for Steam, but I understand why people use it. Personally, I believe someone with deep enough pockets could probably bend Steam/Valve over in court in the US, but it would be a long, grueling trek and not without considerable risk. I for one don't care that Steam exists, I do somewhat care when businesses imply consumers do not have rights that they do actually have in the US. I consider it malicious misdirection in order to further their own selfish interests. Steam is hardly the only company to do this, though, and video games is hardly the only industry with this problem. I'm convinced if libraries hadn't already existed for centuries already people would scream bloody murder and the general populace in the US would think the evil libraries were "stealing" from artists. Thankfully they have existed long enough to be an established tradition.
Just because reselling digital goods doesn't happen a lot right now doesn't mean it won't become more common as the purchase of said goods does. I do not think selling "accounts" is as clear cut of an issue, though, at least not legally (morally I suppose they should either both be "okay" or "not okay", hand in hand). But selling the items one can get from said accounts, should that be possible, is actually legal in the US.