How do people sell in Fortnite skins?

I often see on Instagram lately that people are selling exclusive skins, which are usually only unlockable when you buy a particular phone.

For example, the Honor Skin for which you usually have to buy an Honor P20 but sell for a price of 50 euro a code where to get the skin.

this is not a rip off so most really forgive the skins.

How does this work? I mean they do not buy countless phones, right?

Is forbidden to sell!