How do gift codes from the Xbox marketplace work?

I have noticed that a lot of websites have “keys” that are generated when you gift a friend a fortnite pack on Xbox and then list the key online. I know some people get their skins revoked if the “keys” were purchased with a stolen credit card, but if I used my own debit card to gift these packs to my alt account, would someone purchasing said keys from me be safe?

I would like to do this so people can get starter packs for 1 or 2 extra dollars after epic stops selling them, but I don’t want to screw anyone over.