Sale 1:

Sold for 13 keys

No outpost link

Seller: http://backpack.tf/profiles/76561198029074524/#!/compare/1460505600/1460592000

Buyer: http://backpack.tf/profiles/76561198037635979/#!/compare/1459987200/1460851200

X = 13

Sale 2:

Quicksold for 12 keys + 11 ref

No outpost link

Seller: http://backpack.tf/profiles/76561198098738257/#!/compare/1455004800/1455091200

Buyer: http://backpack.tf/profiles/76561198065138598/#!/compare/1455004800/1455091200

X = 13

Sale 3:

Sold for Smoking Beret

Cannot find Outpost trade

Seller: http://backpack.tf/profiles/76561198125791160/#!/compare/1456473600/1456560000

Buyer: http://backpack.tf/profiles/76561198128249334/#!/compare/1456473600/1456646400

X = 15

Sale 4:

Quicksold for 12 keys

http://www.tf2outpost.com/trade/27965762 - B/O of 12

Seller: http://backpack.tf/profiles/76561198203537236/#!/compare/1458198000/1460937600

Buyer: http://backpack.tf/profiles/76561198155754968/#!/compare/1458284400/1459036800

X = 12

Sale 5:

Sold in about a day for 12 keys

http://www.tf2outpost.com/trade/28007592 - B/O of 14

Seller: http://backpack.tf/profiles/76561198236673170/#!/compare/1459209600/1459468800

Buyer: http://backpack.tf/profiles/76561198139378656/#!/compare/1459123200/1459209600

X = 12

Sale 6:

Sold with ‘Most Wanted’ taunt for Flies Merryweather

No outpost link

Seller: http://backpack.tf/profiles/76561198141308660/#!/compare/1458950400/1459036800

Buyer: http://backpack.tf/profiles/76561198075468799/#!/compare/1458864000/1458950400

X + 0.5 = 12

X = 11

Sale 7:

Quicksold for 11 keys

http://www.tf2outpost.com/trade/27962571 - B/O of 12

Seller: http://backpack.tf/profiles/76561198128249334/#!/compare/1457938800/1458111600

Buyer: http://backpack.tf/profiles/76561198203537236/#!/compare/1457938800/1458111600

X = 11

Sale 8:

Sold for Bubbling Nanobalaclava

No outpost link

Seller: http://backpack.tf/profiles/76561198061410470/#!/compare/1458777600/1458864000

Buyer: http://backpack.tf/profiles/76561198141308660/#!/compare/1458802800/1458864000

X = 11

Sale 9:

Sold for Strange Pro. KS Sniper Rifle + Buildings Destroyed part

http://www.tf2outpost.com/trade/27762081 - B/O of 14

Seller: http://backpack.tf/profiles/76561198131697922/#!/compare/1454918400/1455091200

Buyer: http://backpack.tf/profiles/76561198098738257/#!/compare/1455004800/1455091200

X = 7.5 + 2

X = 10

Sellers:

12.5:

http://www.tf2outpost.com/trade/28101959 - young

14 keys:

http://www.tf2outpost.com/trade/28101494 - young

15 keys:

http://www.tf2outpost.com/trade/28054215 - 2 weeks

Sales at:

15

13

13

12

12

11

11

11

10

Too many quicksales here. Wouldn’t make much sense to exclude everything below 13, though. Seeing as both of the 12 key sales took less than a day to sell, I'm going to exclude 12 and below, and go with 13-15.