Sale 1:

Sold for 20 keys

http://www.tf2outpost.com/trade/28144578 - B/O of 20

Seller: http://backpack.tf/profiles/76561198125869453#!/compare/1462752000/1462838400

Buyer: http://backpack.tf/profiles/76561198132068284#!/compare/1462838400/1463270400

X = 20

Sale 2:

Sold for 17 keys

http://www.tf2outpost.com/trade/28093662 - B/O of 20, seller confirms sale (Legend)

Seller: http://backpack.tf/profiles/76561198056793444#!/compare/1461110400/1461196800

Buyer: http://backpack.tf/profiles/76561198071553799#!/compare/1461110400/1461196800

X = 17

Sale 3:

Sold for Circling Peace Sign Hot Rod

http://www.tf2outpost.com/trade/28172964 - B/O of 22

Seller: http://backpack.tf/profiles/76561198029910109#!/compare/1463529600/1463616000

Buyer: http://backpack.tf/profiles/76561198107039369#!/compare/1463616000/1463702400

X = 17

Sale 4:

Sold for 6 keys + Genuine Friends Forever Companion Square Badge

Seller: http://backpack.tf/profiles/76561198132068284#!/compare/1463270400/1463356800

Buyer: http://backpack.tf/profiles/76561198029910109#!/compare/1463097600/1463356800

X = 12

Sale 5:

Sold for Logo Fruit Shoot

No outpost

Seller: http://backpack.tf/profiles/76561198071553799#!/compare/1462492800/1462579200

Buyer: http://backpack.tf/profiles/76561198125869453#!/compare/1462147200/1462579200

Would prefer to have the Fruit Shoot updated with this sale

Sellers:

20 keys:

http://puu.sh/pKhsE/99c0f3d90a.png

Sales at:

12

17 (2x)

20

12 is quite obviously an outlier

Going with 17-20