Large Birthday Cards I See People Selling Large Value Gift Cards/vouchers On EBay But Where Do They Get Them And R They Legit/real?

I see people selling large value gift cards/vouchers on EBay but where do they get them and R they legit/real? - large birthday cards

For example, I could be a bid of £ 600 of Argos vouchers or £ 100 vouchers and other upcoming auctions only. Sure, some of the activities of limited value (50 pounds or 25 pounds, or any obligations) are clearly undesirable, but nobody made such a large amount of birthday or Christmas, is not it? Stolen? Have you cheated in front of stores. Will I be in danger if I join this? Many people do, and the alert is high enough. The £ 600 of Argos was for something over 450 pounds and is a great savings.
Legal or illegal? Risky or safe?