So, winners are not adding game into library, nor marking it as received on steamgifts

I can kind of understand someone hiding their games if they have activated any Sakura.

For real though, you need to wait the 7 day period. It's valid for both winner and gifter. If they have not marked the game as received after that, you can reroll.

No. You have to give the winner 7 days to activate the game before you can ask support for reroll

Well, by the site rules, they have 7 days (right?) to activate and mark received. So, chill out.

Just because you can see their status as online doesn't mean anything. People leave browsers open, for instance. Don't infer too much.

You can use sgtools to check the winners to see if they have strikes against them or if they have a good record.

If after all that, it turns out that they used the key and didn't provide received feedback, you can report to mods.

Some winners are weird I guess. Someone waited the whole week while being online often and right when I sent the reroll request they activated the game. I don't know, maybe it's to collect blacklists?