Sale 1:

Sold for Telsa Coil Wing Mann (10.5)

No outpost

Seller: https://backpack.tf/profiles/76561198130549187#!/compare/1524096000/1524355200

Buyer: https://backpack.tf/profiles/76561198386044837#!/compare/1523923200/1524355200

X = 11

Sale 2:

Sold for $20.89 on Marketplace.tf

https://marketplace.tf/items/30662;5;u96

Couldn't find item's history.

20.89/1.98 = ~10.6

X = 11

Sale 3:

Sold for 10 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1519776000/1519862400

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1519776000/1519862400

X = 10

Sale 4:

Sold for 9 keys + 25 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1522022400/1522108800

Buyer: https://backpack.tf/profiles/76561198284348086#!/compare/1522108800/1522281600

Bot.tf confirms: https://i.imgur.com/Mso8SXw.png

X = 10

Sale 5:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561197994697258#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198071014143#!/compare/1523836800/1524182400

X = 10

Sale 6:

Sold for 9 keys + 22.22 ref

Seller: https://backpack.tf/profiles/76561198023542389#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198260202848#!/compare/1523059200/1523145600

X = 10

Sale 7:

Sold for 9 keys

https://i.imgur.com/s5CDJju.png - B/O of 9

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198292089991#!/compare/1524528000/1524614400

Bot.tf confirms: https://i.imgur.com/Mso8SXw.png

X = 9

Sales 8 & 9:

Two more bot.tf sales at:

9 keys + 2 ref

9 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198048011821#!/compare/1523577600/1523750400

Gotta be one of them, untraceable though

Couldn't find the other sale.

X = 9 (2x)

Sale 10:

Sold for Strange Purple Confetti Elf Esteem

Seller: https://backpack.tf/profiles/76561198026817880#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198255449573#!/compare/1523145600/1523232000

Can be used on the elf.

Sale 11:

Sold for Aces High Brain-Warming Wear

Seller: https://backpack.tf/profiles/76561198292089991#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198153011059#!/compare/1524528000/1524614400

Probably better to use on the Brain.

Sale 12:

Sold with a key + cosmetic for '72 Bucking Bronco

Seller: https://backpack.tf/profiles/76561198029592186#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198119246388#!/compare/1523491200/1523577600

Might be able to use on the Bronco.

Sale 13:

Sold for Mega Strike Killer Solo

Seller: https://backpack.tf/profiles/76561198214273008#!/compare/1519689600/1519862400

Buyer: https://backpack.tf/profiles/76561198352286528#!/compare/1519776000/1519862400

Should be used on the Solo, if it's not too high there.

Sellers:

10 keys:

https://i.imgur.com/pMkjlTj.png

https://backpack.tf/item/6729826337

12 keys:

https://i.imgur.com/9vq40kC.png

https://backpack.tf/item/4201642851

https://i.imgur.com/BowYx4s.png

https://backpack.tf/item/4448555705

13 keys:

https://i.imgur.com/URZ1KrT.png

https://backpack.tf/item/4470534560

Sales at:

9 (3x)

10 (4x)

11 (2x)

Going with 9-11.