I'm stuck at University here in the UK - no option for leaving for another month or so (lockdown), and my nice fresh Series X sits unopened at home. Meanwhile they're going on Ebay for £700... thats £250... 

I want the console but it doesn't make any difference whether I have it over the next month 'cos I can't use it anyway thanks to pandemic. Am I a disgrace, or is it morally permissible to sell it now? Is it really scalping if I didn't set out with the intention of doing it? Or is it a positive-sum trade as I sell it to a more enthusiastic customer? Or am I blinded by that sweet ebay money, stealing from hardworking parents? Is it any better if it stay at home unused? 

Help me with my moral hygiene? Or crucify me if you must. 

Not only it is "morally permissible" to sell it, its also the smart thing to do.

It's called capitalism. There is nothing to feel ashamed about by doing that. Supply and demand. If supply can't keep up, demand goes up and so does price and you benefit. Would I do it? No, but that's because there are safer and more lucrative ways to "day trade" like with stocks. But I wouldn't even do that, I'm thinking about going in on REITs and get some juicy 10-18% return y/y, but that's another story haha. Some may think you are scum, but that is silly as this practice has been done by businesses since capitalism began, so why are individual humans not allowed to do what businesses do? Nonsense, I say!

Ten Hail Marys should cover it.

Supply and demand, baby. You got the supply. Demand some money!

How are you going to sell it? Is someone selling it on your behalf? Also have you seen them SELLING for 750 or just ASKING for 750?

I've been in both the position of buying and selling consoles at ridiculous prices. I was happy in both cases. Likewise, by selling your console, think of the Christmas you might be making for young Timmy who doesn't want anything else in the world besides an Xbox X.There are probably people on this forum that will hate you for it, but I don't think you should be looking for the approval of others when considering financial gains.

Don't think there is anything wrong with scalping a single system. It's when someone buys up all the systems, or a large chunk of something in short supply, and then scalp. I think that is what people really hate.

Irredeemable the moment you bought an xbox, go on and sell.

The only benefit of getting a new system at launch is to be able to play it from day 1. That's it, really. So in your case the only smart thing to do is to sell it and then buy a new one later.