Using last month

Sale 1:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198259342040#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198085004062#!/compare/1523404800/1523491200

X = 10

Sale 2:

Sold for Australium Ambassador (~9.5)

No outpost

Seller: https://backpack.tf/profiles/76561198080044014#!/compare/1523145600/1523836800

Buyer: https://backpack.tf/profiles/76561198008557913#!/compare/1523577600/1523836800

X = 9.5

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198140969595#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198119835383#!/compare/1523577600/1523923200

X = 7

Sale 4:

Sold for Collector's Eyelander (at least 7)

http://www.tf2outpost.com/trade/29612483 - B/O of 6.5

Seller: https://backpack.tf/profiles/76561198062613620#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198062613620#!/compare/1523059200/1523145600

X = 6.5 (Capped by B/O)

Sale 5:

Quicksold for 5 keys

Seller: https://backpack.tf/profiles/76561198072900461#!/compare/1523750400/1523836800

Buyer: https://backpack.tf/profiles/76561198140969595#!/compare/1523750400/1523836800

X = 5

Sellers:

9 keys:

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

https://backpack.tf/item/4977778368

https://i.imgur.com/41J13Ot.png

https://backpack.tf/item/4955350172

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

https://backpack.tf/item/5985149844

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

https://backpack.tf/item/6080427044

10 keys:

https://i.imgur.com/Utk9N8M.png

https://backpack.tf/item/6079067434

15 keys:

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

https://backpack.tf/item/4953864344

Sales at:

5

6.5

7

9.5

10

4 sellers at 9 keys, some of them have been there for quite a while too. The sales at 9.5 and 10 seem to have occurred before these sellers appeared, and it's unlikely that there will be any more sales occurring at those values for a while. Excluding 9.5 and 10.

5 quicksold, excluding 5.

Going with 6.5-7.