A little help regarding gifting keys

Well, check their account for the game. They have 7 days to activate it. If they have not after 7 days, send in a support ticket

Already encountered such situation once.

You can't. Send it in chat is probably safer (and take screenshots) then if they don't activate it / activate on an alt account, report them.

If they claim fake / duplicate key, I don't know what you could do. Ask support?

Even if you send it by mail, you have still the proof you sent that mail with the timestamp... and whoever from the support can try entering that code and see that is perfectly valid...

I am ashamed ppl actually do this anyway... You're winning a free game, take it and shut up.. NO.. they must give troubles even in such situation...

Contacting support seems like the best choice, the more proof you can include (like where the key was gotten from, possibly a screenshot (blocking/editing out any other keys or other private info) will make it look far less likely that you're the one in fault as well.

We recommend not sending keys through e-mail for public giveaways. Instead, send them an e-mail asking them to contact you through Steam to receive it. This allows you to request a re-roll if something goes wrong. Usually, the only problem you'll encounter is someone trying to use it to trade or give to someone else. In such cases, they usually get a suspension and nothing bad happens to you. If they get upset and try to give you 'not received' feedback for some reason, we can change that manually, though it can take a while sometimes. In the worst-case scenario where they try to activate it on another account and claim it's used or something, you usually know whether it should be used or not. In those cases, I recommend contacting the retailer from which you purchased the game and tell them it's been used and you need a new one, and their support should revoke the original one and issue you a new one eventually, allowing you to complete the process and foiling any attempt on their part to scam you out of your key. Please try to save any e-mails or other correspondence with the winner, key retailers, or any other relevant parties, as you can present these to us in support in the worst case to help get things sorted out more quickly and easily.

If the winner its a lier and you are honest,you can proove that you buy the key and send to him.Also you can check his account for that game