Sale 1:

Sold for Green Confetti Dragonborn Helm (15.5) + 1 key

http://www.tf2outpost.com/trade/29177592 - No B/O

Seller: https://backpack.tf/profiles/76561198267189538#!/compare/1494633600/1494720000

Buyer: https://backpack.tf/profiles/76561198324455724#!/compare/1494633600/1494720000

X = 17

Sale 2:

Sold for Aussie Flamethrower (14.4) (other items in the group weren’t involved)

No B/O’s on outpost.

Seller: https://backpack.tf/profiles/76561198164651434#!/compare/1496620800/1496793600

Buyer: https://backpack.tf/profiles/76561198092392401#!/compare/1496707200/1497052800

X = 14

Sale 3:

Sold for $28.7 on Marketplace.tf

https://marketplace.tf/items/30093;5;u57

Buyer: https://backpack.tf/profiles/76561198116132079#!/compare/1490745600/1491264000

https://backpack.tf/item/3632666230

28.7/2.1 = 13.666

X = 14

Sale 4:

Sold for Starstorm Slumber Spirit of the Bombing Past (11) in a day or two

http://www.tf2outpost.com/trade/28884013 - B/O of 12.75

Seller: https://backpack.tf/profiles/76561198110492742#!/compare/1492646400/1492732800

Buyer: https://backpack.tf/profiles/76561198267189538#!/compare/1492646400/1492732800

X = 11

Sale 5:

Sold for 10 keys

No outpost

Seller: https://backpack.tf/profiles/76561198160425427#!/compare/1492214400/1492300800

Buyer: https://backpack.tf/profiles/76561198060358262#!/compare/1492041600/1492300800

X = 10

Sale 6:

Sold in ~3 days for 10 keys + 5 ref

http://www.tf2outpost.com/trade/28880050 - B/O of 11

Seller: https://backpack.tf/profiles/76561198060358262#!/compare/1492387200/1492560000

Buyer: https://backpack.tf/profiles/76561198110492742#!/compare/1492473600/1492560000

X = 10

Sale 7:

Sold for a lot of items.

http://www.tf2outpost.com/trade/29179433 - B/O of 13

Seller: https://backpack.tf/profiles/76561198324455724#!/compare/1494633600/1494720000

Buyer: https://backpack.tf/profiles/76561198152204266#!/compare/1494633600/1494720000

Best not to get an exact figure with this sale.

Sellers:

13 keys:

http://i.imgur.com/fhefumn.png - 22 days

https://backpack.tf/u/76561198012613726

15 keys:

http://www.tf2outpost.com/trade/29227103 - 1 week, re-listed multiple times

https://backpack.tf/u/76561198031569942

Sales at:

10 (2x)

11

14 (2x)

17

10 sold pretty quickly one of the two times, and R+ubi is paying more than 10 here:

https://docs.google.com/spreadsheets/d/1zOs-0OfVjLsNYAmMa8_tvi2tA9Rrnok7nEUqDOQuls0/edit#gid=0

Excluding 10.

11 looks like an outlier, and also sold in a short amount of time. Excluding 11.

17 is another outlier, and would most likely be much lower if we could identify the B/O. Excluding 17.

Aged seller at 13.

Going with 13-14.