Found this by accident. Sale used in the previous suggestion is now outdated and was a quicksale anyways.

http://www.tf2outpost.com/trade/28483607 ----- Quickbuy trade where the previous owner offered his hat for 18 keys.

Sale 1

Sold for 25 keys (sort by time)

http://backpack.tf/profiles/76561198042938501#!/compare/1480896000/1481068800

http://backpack.tf/profiles/76561198335686534#!/compare/1480723200/1481414400

http://www.tf2outpost.com/trade/28547779 ----- B/O was 25 keys, took a while to sell.

Keys don't match with the buyer due to a big inventory history gap. A trade happened between the buyer and "孤寂老仙" (the one who appears in key histories before Evil), as seen here:

http://backpack.tf/item/3729182287

=======> 25 keys

======================================================

Sellers

- No sellers at the time.

Summary

- Sale for 25.

Going with 25 keys. Criticism appreciated!