Sale 1:

Sold for $25.95 on Marketplace.tf

https://marketplace.tf/items/109;5;u35

https://backpack.tf/item/1460788162

25.95/2.07 = ~12.54

Going to round down, as pure sales are pretty common in the 10-12 key range.

X = 12

Sale 2:

Sold for 12 keys

No outpost

Seller: https://backpack.tf/profiles/76561197967286520#!/compare/1499040000/1499126400

Buyer: https://backpack.tf/profiles/76561198299605097#!/compare/1499040000/1499126400

X = 12

Sale 3:

Sold with cheap cosmetic for 12 keys

No outpost

Seller: https://backpack.tf/profiles/76561198363320256#!/compare/1493942400/1494028800

Buyer: https://backpack.tf/profiles/76561198157031925#!/compare/1493942400/1494028800

X = 12

Sale 4:

Sold for 11 keys

No outpost

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1496966400/1497052800

Buyer: https://backpack.tf/profiles/76561198258444096#!/compare/1496966400/1497052800

X = 11

Sale 5:

Sold for 11 keys (Buyer’s compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1499904000/1499990400

Buyer: https://backpack.tf/profiles/76561198055056456#!/compare/1499644800/1499990400

X = 11

Sale 6:

Sold for 10 keys in ~2-3 days

No outpost

Seller: https://backpack.tf/profiles/76561198144346135#!/compare/1500336000/1500422400

Buyer: https://backpack.tf/profiles/76561198258264645#!/compare/1500249600/1500422400

X = 10

Sale 7:

Sold for 10 keys + <0.5 keys in ref (Buyer’s compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1495929600/1496016000

Buyer: https://backpack.tf/profiles/76561198119752775#!/compare/1495929600/1496016000

X = 10

Sale 8:

Sold for 10 keys in ~4 days

http://www.tf2outpost.com/trade/29209078 - B/O of 10

Seller: https://backpack.tf/profiles/76561198119752775#!/compare/1496016000/1496620800

Buyer: https://backpack.tf/profiles/76561197997798761#!/compare/1496188800/1496620800

X = 10

Sale 9:

Sold with Strange Pro. KS Shotgun for Blizzardy Fro

Seller: https://backpack.tf/profiles/76561198258264645#!/compare/1500681600/1500768000

Buyer: https://backpack.tf/profiles/76561198088904542#!/compare/1500681600/1500768000

Should be used on the Fro

Sale 10:

Sold with 1 key + <0.5 keys in ref for Cloud 9 Ye Oiled Baker Boy

Seller: https://backpack.tf/profiles/76561198055056456#!/compare/1499644800/1499990400

Buyer: https://backpack.tf/profiles/76561198101649245#!/compare/1499817600/1500076800

Should be used on the Baker Boy.

Sale 11:

Sold for Starstorm Slumber Wing Mann + 1 key + <0.5 keys in ref

No outpost

Seller: https://backpack.tf/profiles/76561197997798761#!/compare/1496620800/1496707200

Buyer: https://backpack.tf/profiles/76561198038102263#!/compare/1496534400/1496707200

Should fall into the range.

Sellers:

15 keys:

http://www.tf2outpost.com/trade/29286529 - 6 days

Sales at:

10 (3x)

11 (2x)

12 (3x)

Sales at 10 are far too common to be excluded.

Going with 10-12.