Gave him the key, and he didn't marked as received:(

Wait till the 7 Days are up. Then Report him. And there are so many of these Topics, Please use the Search button next time :) .

Did he already activate it?

Wait a bit then...

He might have activated it, Dota doesn't show in libraries until you install and play.

Can't you ask the winner to mark it instead complain in here?

Make contributor giveaways. Generally people who have gifted in the site know how it works and won't screw you over.

If you're sure he hasn't activated it, request a reroll, give the new winner the key. :p

7 days, be patient. I got emailed a key from a giveaway I won, and only checked email 2 days later, thankfully the contributor was patient. It needs to work both ways

1. This topic is spam. Read the FAQ instead of asking us what it says.
2. You're not allowed to give away a DOTA 2 key.

Since I read the FAQ, I know that I should report you for both of these things, so please close this spam thread before anyone reports you. Personally, I don't care about rule #2, so I won't bother reporting for that, but I am annoyed by the spam and will report you for that if the topic stays active.

It's a DOTA 2 key. It's not even allowed. The only legal DOTA 2 giveaways are tradable copies.

Perhaps that's why he didn't mark it as received?