Diplomat

http://backpack.tf/item/1327545148

Sale 1) Sold for 11 Keys

http://backpack.tf/profiles/76561198311227374#!/compare/1492214400/1493251200 - Buyer's Backpack

Sale 2) Sold + 1 Key for an Aces High Airdog (14 Keys)

http://backpack.tf/profiles/76561198311227374#!/compare/1493251200/1493424000

14 - 1 = 13 Keys

Sale 3) Sold for 10 Keys + ~1 Key of Items

http://backpack.tf/profiles/76561198260126432#!/compare/1493596800/1493683200

10 + 1 = 11 Keys

http://backpack.tf/item/4161029236

Sale 4) Sold + ~2 Keys of Items for a Kill-a-Watt Sober Stuntman + ~1 Key of Items

http://backpack.tf/profiles/76561198084806911#!/compare/1493164800/1493251200

Fits in the range.

Sale 5) Sold for an Orbiting Fire Larrikin Robin (15 Keys)

http://backpack.tf/profiles/76561198175249965#!/compare/1494028800/1494115200

Sale 6) Sold for 10 Keys

http://backpack.tf/profiles/76561198119057660#!/compare/1494374400/1494460800 - Buyer's Backpack

Counting this low, but using it as a supportive sale for a drop.

Sale 7) Sold for a Massed Flies Grenadier's Softcap (11.5 Keys) + 3 Keys

http://backpack.tf/profiles/76561198119057660#!/compare/1494460800/1494547200 - Seller's Backpack

http://backpack.tf/profiles/76561198260811500#!/compare/1494460800/1494547200 - Buyer's Backpack

11.5 + 3 = 14.5, which rounds to 15 Keys

Softcap Mini

http://backpack.tf/profiles/76561198129180692#!/compare/1493596800/1493683200 - Sold for a Bubbling Bear Necessities (~13 Keys)

http://backpack.tf/profiles/76561198269650829#!/compare/1493683200/1493942400 - Sold for 10 Keys