Sale 1:

Sold for 18 keys

http://www.tf2outpost.com/trade/28906820 - No B/O

Seller: https://backpack.tf/profiles/76561198126204929#!/compare/1494288000/1494374400

Buyer: https://backpack.tf/profiles/76561198054218083#!/compare/1493942400/1495670400

X = 18

Sale 2:

Sold for 18 keys

http://www.tf2outpost.com/trade/28807839 - No B/O

Seller: https://backpack.tf/profiles/76561198076020691#!/compare/1490918400/1491004800

Buyer: https://backpack.tf/profiles/76561198271204327#!/compare/1490832000/1491004800

X = 18

Sale 3:

Sold for 16 keys in ~7 days

No outpost

Seller: https://backpack.tf/profiles/76561198344491537#!/compare/1493078400/1493164800

Buyer: https://backpack.tf/profiles/76561198170829878#!/compare/1493078400/1493164800

X = 16

Sale 4:

Sold for Purple Confetti Pirate Bandana (16)

No outpost

Seller: https://backpack.tf/profiles/76561198209232909#!/compare/1494115200/1494547200

Buyer: https://backpack.tf/profiles/76561198056866244#!/compare/1494460800/1494547200

X = 16

Sale 5:

Quicksold for 15 keys (Buyer’s compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198267189538#!/compare/1491696000/1491782400

Buyer: https://backpack.tf/profiles/76561198114517376#!/compare/1491696000/1491782400

X = 15

Sale 6:

Sold for 15 keys in ~4 days

No outpost

Seller: https://backpack.tf/profiles/76561198114517376#!/compare/1492300800/1492732800

Buyer: https://backpack.tf/profiles/76561198204575909#!/compare/1492560000/1492646400

X = 15

Sale 7:

Sold for ~15 keys in items in ~5 days

No outpost

Seller: https://backpack.tf/profiles/76561198204575909#!/compare/1492732800/1492992000

Buyer: https://backpack.tf/profiles/76561198011321852#!/compare/1492905600/1492992000

X = 15

Sale 8:

Sold for 15 keys

No outpost

Seller: https://backpack.tf/profiles/76561198027694776#!/compare/1498003200/1498089600

Buyer: https://backpack.tf/profiles/76561198059927802#!/compare/1497916800/1498176000

X = 15

Sale 9:

Sold for 15 keys + 12 ref (<0.5) in ~10 days (Buyer’s compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1492992000/1493078400

Buyer: https://backpack.tf/profiles/76561198055774847#!/compare/1492992000/1493078400

X = 15

Sale 10:

Sold for Nuts n Bolts Killer’s Kabuto + Strange Festive Ambassador (<4)

http://www.tf2outpost.com/trade/28906333 - B/O of 18

Seller: https://backpack.tf/profiles/76561198068200165#!/compare/1493942400/1494028800

Buyer: https://backpack.tf/profiles/76561198131040073#!/compare/1493942400/1494028800

Will either cap at the B/O or fall just short of it. Either way it fits in the range.

Sale 11:

Bulk for Hellfire Halogen Head Lamp

Seller: https://backpack.tf/profiles/76561198055774847#!/compare/1493337600/1493424000

Buyer: https://backpack.tf/profiles/76561198068200165#!/compare/1493337600/1493424000

Sale 12:

Bulk for Secret Front Runner

Seller: https://backpack.tf/profiles/76561198056866244#!/compare/1494892800/1494979200

Buyer: https://backpack.tf/profiles/76561198104098977#!/compare/1494892800/1494979200

Sale 13:

Bulk for Mystery Pullover

Seller: https://backpack.tf/profiles/76561198104098977#!/compare/1495584000/1495756800

Buyer: https://backpack.tf/profiles/76561198027694776#!/compare/1495497600/1495756800

Sellers:

17 keys:

http://i.imgur.com/dfx5WbB.png - 1 month

https://backpack.tf/u/76561198344491537

https://backpack.tf/item/293527025

18 keys:

http://i.imgur.com/UBye7wD.png - young

https://backpack.tf/u/76561198059927802

http://www.tf2outpost.com/trade/29241028

Sales at:

15 (5x)

16 (2x)

18 (2x)

15 may have quicksold a few times, but it sold far too many times to be ignored.

Going with 15-18.