Illegal, because you're violating copyright law by pirating it, but not amoral, because you are harming nobody and not acting against the underlying purpose of copyright law. You've compensated the developer by purchasing the game, so there is no de facto difference between you playing the legitimate copy or a pirated one, even if there is a de jure difference. Retaining resale value is bringing no harm to the developers or owners of the IP, and seems to be a distraction from the underlying issue being discussed. If we remove this from the scenario, and say "is it OK to download a pirated version of a game I already own for convenience sake", we get more clearly to the crux of the issue.
I think the interesting point here is the morality. The legality is in such grey, boring shades that it's really only up to finding the right court-case to say if this is legal or not. My understanding of things is that you're allowed to have a backup of your game.
So let's focus on that. I know that in my country, it's only illegal to upload copy-righted material. This means that downloading the game in itself is not illegal - don't know if this is the case for the US. So, let's assume it's legal to download a game to back it up when you own it. A good example here is Wii-games. Say I want to have a backup of Xenoblade on a harddrive. I can't rip my own game, since my DVD-drive can't read Wii-discs. So I download it, and use the rip instead. Should I then be able to take a "back up" if my disc has already been destroyed? Say I own the game, but I come home and step on it. Bah! I should've backed up up yesterday. The internet to the rescue. Should be OK, right? I still own the license to the things on the disc, and it's just the disc itself that's been damaged. In that case, it's OK to download a game you can't access.
In this case, the sealed case is preventing OP from accessing the disc, but he still owns it. Depending on how we view the previous paragraph, it's legal to download a copy of a game he owns but cannot access.
And that's not immoral, either. How about, then, when the OP does it to sit on it for years, so it becomes valuable, then sell it as a sealed copy, whereas someone else who did not do this morally bending backwards has to buy two copies - one to play, and one to keep for collector's value. You've cut the corner on the whole thing making those copies worth money. It's the fact that they've never been played. Say the next Xbox came out, and it could read discs through the case. You'd never have to break the seal of another case, and the value would just disappear.
In that, I think there's a display of immorality. It's a cheaters way of having an unopened game. Unopened implies "hasn't been played" - but clearly the license has been played, since it's that license that you justify the downloading with. It's like a virgin having been a phone-sex operator for years. He/she's technically still a virgin, but what the fuck.
It's immoral to resell it later, because you're implying it has a value of never having been used, while it still served its purpose to allow you to play. Until the rest of the collector's market catches on, it's really only scamming collectors, by not having to put up the same value as others selling the game "morally".
It's highly debatable, and this is way too loosely written. But the core is there - and some moral relativist out there can probably put it to better terms than I did.