I had before me to buy a skin at G2A.
However, I'm afraid that I will not get the key or the key is wrong.
That would be the Honor Guard Skin.
The seller has a rating of 98%.
https://www.g2a.com/fortnite-honor-guard-skin-epic-games-key-global-i10000187101001
It's a good seller. Also gives a "money back guarantee" if it does not work.