a proof of the trade would be awesome (that that sale took place)

http://www.tf2outpost.com/trade/21894050

There is another sale. You cannot price this based on one sale since the following criteria is not fully satisfied.

http://forums.backpack.tf/index.php?/topic/6033-new-unusual-price-suggestion-guide-the-cheat-sheet/

Exception 2: Rare Hats

Any hat can be priced based on a single sale if ALL these criteria are met:

- There are no other recent sales in the last 3 months

- The effect is not a summer 2013 effect. (robo, 2013 halloween effects can be used as they are often rarely traded and rarely unboxed)

- The hat went to a collector or person who intends to keep the hat. A collector can be identified by looking at the person's trade history, item history of similar unusuals in the backpack OR if the owner has not resold the hat 1 month after the sale occurred.

1 on the market therefore you need atleast 2 sales

Thanks to me, it came to exist because I uncrated it: http://steamcommunity.com/sharedfiles/filedetails/?id=293219972

I sold it for 2 Earbuds pure but it was 1½ months ago so It's no longer valid proof.