Sale 1:

Sold for 11 keys

No outpost link available

Seller: http://backpack.tf/profiles/76561198074924284/#!/compare/1456732800/1456819200

Buyer: http://backpack.tf/profiles/76561198207940136/#!/compare/1456732800/1456819200

Clearest compare links ever

X = 11

Sale 2:

Sold for 10 keys

http://www.tf2outpost.com/trade/27853464 - B/O of 11

Seller: http://backpack.tf/profiles/76561198028729051/#!/compare/1455091200/1456819200

Buyer: http://backpack.tf/profiles/76561198044315099/#!/compare/1455868800/1456300800

X = 10

Sale 3:

Sold for 8 key + 6 ref

http://www.tf2outpost.com/trade/27724705 - B/O of 8

Seller: http://backpack.tf/profiles/76561198126159690/#!/compare/1450512000/1453104000

Buyer: http://backpack.tf/profiles/76561198080179568/#!/compare/1453017600/1453104000

I do believe the buyer was already buying for 8.3 keys, and the seller created the trade before seeing the buy order, which is why extra was paid.

X = 8.3

Rounding to 8

Sellers:

10 keys:

http://puu.sh/nJJjK/410ddd9c7d.png - 2 days unbumped

Buyers:

8.6 keys:

http://www.tf2outpost.com/trade/27915218

Sales at:

8 (Pure)

10 (Pure)

11 (Pure)

8 is an outlier

Going with 10-11