what do i do if

I believe if it lasts like that for a while you can send support a ticket explaining your situation to them and they can help you out. The only other thing I can suggest is if he/she doesn't speak English very well you can try finding out what language they do speak and maybe translate the message to them?

Just wait for a few more days and if they still don't mark as received you can contact support along with proof like a screenshot.

Or you can guide them with some screenshots? Like teaching a kid reading a book, they always go for the pictures first. It's easier to understand with images than words.
Also, use simpler words.

Execute them for HERESY!

I dunno, it's only been a day...wait a while longer. I'm not always immediately able to activate a game I win on here, so I have to wait to mark it as received

just to update: this guy still hasnt left feedback for me. i took screenshots of our steamchat and opened a support ticket. thanks for your help!

im actually having another issue now, where i tried to add a winner on steam (since the gift is in my inventory) and he blocked me on steam... lol

When I encounter a problem like this I always pray at my Onion Shrine. Every problem I had is all ogre now, however I've never heard from the troublemakers themselves again.