Fireball.

http://backpack.tf/item/988677467

Sale 1) Sold for a Bubbling Backwards Ballcap (16.5 Keys) + ~2 Keys in Items + 1 Key + ~0.35 Keys of Refined

http://backpack.tf/profiles/76561198080179568#!/compare/1492732800/1492819200

16.5 + 2 + 1 + .35 = 19.85, which rounds to 20 Keys

Sale 2) Sold for 16 Keys + ~0.5 Keys of Items

http://backpack.tf/profiles/76561198334755696#!/compare/1493164800/1493251200

16 + 0.5 = 16.5, which rounds to 17 Keys

Sale 3) Sold + Starduster (2.55 Keys) for a Searing Plasma Thirst Blood

http://backpack.tf/profiles/76561198035312409#!/compare/1493164800/1493251200 - Buyer's Backpack

http://www.tf2outpost.com/trade/28896754 - Capped by the Buyout of 20 Keys

http://www.tf2outpost.com/trade/28900245 - 1 Week Seller at 20 Keys