Swashbuckling.

https://backpack.tf/item/6538675001

Sale 1) Sold for a Steaming Hustler's Hallmark (9.5 Keys) + ~5.7 Keys of Items

https://backpack.tf/profiles/76561198081793950#!/compare/1529539200/1529798400

9.5 + 5.7 = 15.2, which rounds to 15 Keys

Sale 2) Sold + ~7 Keys of Items for a Blizzardy Storm Sergeant's Drill Hat

https://backpack.tf/profiles/76561198142482594#!/compare/1530144000/1530230400

This sale would probably be better used for the Drill.

Sale 3) Sold for 18 Keys

https://backpack.tf/profiles/76561198021055079#!/compare/1531267200/1531353600

https://backpack.tf/item/5394569761

Sale 4) Quicksold for 14 Keys + ~0.5 Keys of Items

https://backpack.tf/profiles/76561198182456374#!/compare/1531267200/1531353600

Sale 5) Bulk sale

https://backpack.tf/profiles/76561198317606171#!/compare/1532217600/1532304000

https://backpack.tf/profiles/76561198170374989#!/compare/1532131200/1532304000

Sale 6) Sold for 18 Keys

https://backpack.tf/profiles/76561198170374989#!/compare/1533945600/1534032000