Trading

You must activate your winnings to your own account.

"You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account used during registration. If a friend is interesting in joining giveaways, they'll need to register for their own account."
You have to redeem it, no exceptions. Any other questions you may have? :)

From the FAQ - 'Winnings must be redeemed on the winning account.'

if you win a key here, you have to activate it or face suspension.

Can I enter a giveaway for a friend?
You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account used during registration. If a friend is interesting in joining giveaways, they'll need to register for their own account.

http://www.steamgifts.com/about/faq

Why is that rule set that way although?

Why don't they make a rule that forbids REGIFTING the same thing like if you use the same key you got in a giveaway your account is banned instead of forbidding Trading/Gifting keys to anyone. That they check if the key has been activated and that it has to be activated in 72 hours or the key will be taken from you or you will be banned. Let's say.

It's not allowed to trade them in here or anywhere else as you;re required to activate your winnings otherwise you'll get yourself blacklisted by users & suspended from Sg.