This is my first suggestion, so it's probably going to look weird.

I sold my hat - after 5 years of sitting on it - to this gentleman: http://steamcommunity.com/profiles/76561198094365132 - for 420 Keys.

The other two that exist (to my knowledge) have been traded around quite a bit at this point, but mine's trading hands for the first time today.

As the sole owner of such a unique hat, this was about the price I was hoping for for this thing. I was aiming for something closer to 500+ Keys, but I was more than happy with this person's offer. It took getting through a lot of people, but I finally found someone I was happy to hand it over to.

I'd be happy for someone else to make a price range suggestion. I just thought I'd put it out there that I sold mine for 420.

Mine doesn't have any trade history so it's marginally different to the other two.

I don't know how I can provide proof, let me know if there's anything I can provide. Have a nice day!

This is just an image of the confirmation screen on my mobile device, it's all I have for now: https://i.imgur.com/wGx7Lf5.jpg