Sale 1:

Sold for 13 keys in ~1 day

No outpost

Seller: https://backpack.tf/profiles/76561198040224815#!/compare/1504310400/1504396800

Buyer: https://backpack.tf/profiles/76561198366335650#!/compare/1504051200/1504396800

X = 13

Sale 2:

Sold for Bubbling Defiant Spartan (In-date at 13, month-old seller at <12. Taking <12) + Spec. KS Frying Pan (~1)

http://www.tf2outpost.com/trade/29335699 - No B/O

Seller: https://backpack.tf/profiles/76561198132226518#!/compare/1503187200/1503273600

Buyer: https://backpack.tf/profiles/76561198107278593#!/compare/1503100800/1503273600

X = 13

Sale 3:

Sold for 12 keys

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1506124800/1506211200

Buyer: https://backpack.tf/profiles/76561198146343034#!/compare/1506211200/1506297600

X = 12

Sale 4:

Sold for 10 keys + 4 ref

http://www.tf2outpost.com/trade/29355339 - No B/O

Seller: https://backpack.tf/profiles/76561198009384764#!/compare/1503360000/1504310400

Buyer: https://backpack.tf/profiles/76561198040224815#!/compare/1504224000/1504310400

X = 10

Sale 5:

Bulk for Stormy 13th Hour Rubber Glove

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1498608000/1498694400

Buyer: https://backpack.tf/profiles/76561198321713557#!/compare/1498608000/1498694400

Sale 6:

Sold for 9 keys + a lot of ref + possibly buds

No outpost

Seller: https://backpack.tf/profiles/76561198231082180#!/compare/1503273600/1503360000

Buyer: https://backpack.tf/profiles/76561198106865190#!/compare/1503187200/1503360000

Can’t confirm if the buds were involved, as they don’t match. Ignoring this sale.

Sellers:

15 keys:

https://i.imgur.com/tuLDNXE.png - 6 months

Sales at:

10

12

13 (2x)

10 looks low. Excluding 10.

Going with 12-13.