Josh Ling @ Adopt Me!

I honestly am not sure, although given they always sell new PC games super cheap, I would imaaagine they are not super legal/moral in what they do haha

Thats been my assumption. TBH, I only tend to do it for huge games from huge studios, because for example, I'm not paying £50 for Monster Hunter. (I still haven't bought MHW yet, but it's an example)

Whereas if it's an indie game I'm interested in or been following, I'll buy from Steam/GoG, or sometimes from the Dev.

You should buy all games from licensed retailers. It's more expensive, yes, because that's what the Devs decided the game costs. If you don't want to pay, wait for a sale or don't play it. If you're gonna buy resold keys might as well pirate tbh, same amount of money goes to devs

A resold key has been supplied from somewhere, so some money should have went to the dev. Also, nobody personally involved in making AAA titles chooses the RRP, and £50 is a lot of money, especially when AAA games are often some really bad PC ports.

This is a bad justification. Buying resold keys is immoral regardless of the size of the studio you're buying from. If the asking price is too much then you can't afford to play it, what you're doing is only slightly better than actual piracy

Should I also abstain from buying pre-owned games for my console from a physical store?

Good Q. I'm not sure I understand the differences in this.