Is it possible that the winner of a key will mark the key as "unrecieved" even if he got it?

If the winner did redeem it, then it will be obvious. Not so much if he fed it to an alt, but you could still try to defend yourself with chat logs if you still have them.

Yes, if he already had the game and asked the owner to reroll it and the owner never did.

Edited: Only read the guidelines

This is why a lot of people limit their giveaways to Level 1. It weeds out all the bots who harvest keys and don't bother to leave feedback.

They could if they are dishonest.

It's one of those risk you have to take since this is all based on the honor system.That only option you really would have is if you had access to what account used said key.This is one thing I missed about how humble use to do key redemption is that it would show the steam account that redeemed the product if it was sent as a gift as it did not issue keys but a steam account to redeem it if memory servers me well.

You can do things to help avoid it but you will still have no way of knowing as even people with higher levels have been knowing to re-gift,win same game twice and so on.It's kind of like Ebay where the person receivingit has more leverage then the seller due to being harder to prove said product was working or the right product for that matter.