Sales 1 & 2:

Sold for $19.8 each

https://marketplace.tf/items/613;5;u35

https://backpack.tf/item/1311156057

Can't identify the other one.

19.8/2.04 = ~9.7

19.8/2.03 = ~9.75

X = 10 (2x)

Sale 3:

Sold for Bubbling Elf Esteem (9.5) + 5 ref

No outpost

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1502150400/1502236800

Buyer: https://backpack.tf/profiles/76561198144234206#!/compare/1502150400/1502236800

X = 10

Sale 4:

Sold for 8 keys + 16 ref in ~2 days (Buyer's compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1498089600/1498176000

Buyer: https://backpack.tf/profiles/76561198150653411#!/compare/1498089600/1498176000

X = 9

Sale 5:

Sold for 9 keys in ~1 day

Seller: https://backpack.tf/profiles/76561198267189538#!/compare/1502064000/1502150400

Buyer: https://backpack.tf/profiles/76561198146457260#!/compare/1502064000/1502150400

X = 9

Sale 6:

Quicksold for 8 keys

Seller: https://backpack.tf/profiles/76561198080253538#!/compare/1499212800/1499299200

Buyer: https://backpack.tf/profiles/76561197996904846#!/compare/1499212800/1499299200

X = 8

Sale 7:

Bulk for Ghosts Tossle

Seller: https://backpack.tf/profiles/76561198165576105#!/compare/1499212800/1499299200

Buyer: https://backpack.tf/profiles/76561198080253538#!/compare/1499212800/1499299200

Sellers:

~9.3 keys:

http://i.imgur.com/2NcZEzJ.png - 2 days

~9.55 keys:

http://i.imgur.com/MUpQJ0y.png - 4 days

10 keys:

http://www.tf2outpost.com/trade/29278128 - 1 month

Sales at:

8

9 (2x)

10 (3x)

8 has no place here. Excluding 8.

Sales at 9 may have been quicksells, however, the month old seller at 10 keys, and the sellers for less than 10 show that 10 flat will not work here.

Going with 9-10.