Using last 2 months.

Sale 1:

Sold for Dead Presidents Rubber Glove (Mini at 10 below)

No outpost

Seller: https://backpack.tf/profiles/76561198166985758#!/compare/1522972800/1523145600

Buyer: https://backpack.tf/profiles/76561198027192131#!/compare/1522800000/1523145600

X = 10

Sale 2:

Sold for 10 keys

No outpost

Seller: https://backpack.tf/profiles/76561198027192131#!/compare/1523145600/1523318400

Buyer: https://backpack.tf/profiles/76561198211995121#!/compare/1523232000/1523318400

X = 10

Sale 3:

Sold for 10 keys + 11 ref to a quickbuyer

Seller: https://backpack.tf/profiles/76561197972383519#!/compare/1525046400/1525132800

Buyer: https://backpack.tf/profiles/76561197995029705#!/compare/1525046400/1525132800

X = 10

Sale 4:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198054833725#!/compare/1525996800/1526083200

Buyer: https://backpack.tf/profiles/76561198127395094#!/compare/1525651200/1526083200

X = 9

Sale 5:

Sold for Orbiting Planets Western Wear?

Seller: https://backpack.tf/profiles/76561198396471883#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198337902688#!/compare/1523664000/1523750400

The Wear was traded back, but nothing else appears to have been traded back with it. Probably better to not use this sale.

Sale 6:

Bulk for Insomnia Cone

Seller: https://backpack.tf/profiles/76561198140010255#!/compare/1522540800/1522972800

Buyer: https://backpack.tf/profiles/76561198305458404#!/compare/1522454400/1522972800

Dead Presidents Glove Mini:

Sold for 10 keys (some keys worth more than others) + 3 ref

Seller: https://backpack.tf/profiles/76561197961080670#!/compare/1520812800/1522368000

Buyer: https://backpack.tf/profiles/76561198208966079#!/compare/1520726400/1523318400

X = 11

2 bot.tf sales at 9 keys + some ref:

https://i.imgur.com/xYnBC9J.png

https://backpack.tf/item/3339715572

https://backpack.tf/profiles/76561198304138933#!/compare/1524787200/1524873600

https://backpack.tf/item/1853355380

https://backpack.tf/profiles/76561198304138933#!/compare/1524355200/1524441600

Taking 9-11 (10)

Sellers:

12 keys:

https://i.imgur.com/DBmvcI9.png

https://backpack.tf/item/3317311240

~12.5 keys:

http://www.tf2outpost.com/trade/29650566

16 keys:

https://i.imgur.com/g9DEtHH.png

https://backpack.tf/item/6544681832

Sales at:

9

10 (3x)

10 may have sold to a quickbuyer once, but there are currently no quickbuyers at 10 keys, and there are no sales for higher.

https://i.imgur.com/XB6bZsr.png - Buyers at 9, excluding 9.

Going with 10 flat.