Sale 1:

Sold for 36 keys

http://www.tf2outpost.com/trade/29228386 - B/O of 36

Seller: https://backpack.tf/profiles/76561198278732896#!/compare/1497484800/1497571200

Buyer: https://backpack.tf/profiles/76561198070880686#!/compare/1497484800/1497571200

X = 36

Sale 2:

Sold for $70.20 on Marketplace.tf

https://marketplace.tf/items/54;5;u36

https://backpack.tf/item/906903364

70.2/2.05 = ~34.4

X = 34

Sale 3:

Sold for $60 on Marketplace.tf

https://marketplace.tf/items/54;5;u36

https://backpack.tf/item/1118914301

60/2.05 = ~29.3

X = 29

Sale 4: (Dupe)

Sold for Stormy Storm Executioner (28)

http://www.tf2outpost.com/trade/29204597 - No B/O

Seller: https://backpack.tf/profiles/76561198156232211#!/compare/1498348800/1499299200

Buyer: https://backpack.tf/profiles/76561198082830143#!/compare/1498867200/1499212800

X = 28

Sale 5:

Bulk for Burning Chieftain's Challenge

Seller: https://backpack.tf/profiles/76561198095533997#!/compare/1497312000/1497398400

Buyer: https://backpack.tf/profiles/76561198278732896#!/compare/1497312000/1497398400

Sale 6:

Sold for Steaming Tipped Lid + 2 keys

No outpost

Seller: https://backpack.tf/profiles/76561198260967101#!/compare/1501977600/1502064000

Buyer: https://backpack.tf/profiles/76561198087172601#!/compare/1501977600/1502064000

Better off being used on the Tipped Lid. Couldn't find many sales that weren't quicksales.

Sale 7: (Dupe)

Sold for Duped Ghosts Samur-Eye

No B/Os on outpost

Seller: https://backpack.tf/profiles/76561198125791160#!/compare/1495843200/1495929600

Buyer: https://backpack.tf/profiles/76561198156232211#!/compare/1495324800/1495929600

Not using this sale without a B/O.

Sale 8: (Dupe)

Sold with ~12 keys in items for Duped Sulphurous Trilby

Seller: https://backpack.tf/profiles/76561198082830143#!/compare/1499299200/1499385600

Buyer: https://backpack.tf/profiles/76561198086948995#!/compare/1499126400/1499299200

Pretty bulky, not using here.

Sellers:

32 keys:

http://i.imgur.com/vFHCKTw.png - 12 days

42 keys: - Duped

http://i.imgur.com/g3LDhMw.png - 2 months

Sales at:

28 (Duped)

29

34

36

Excluding 28, because it's duped.

Going with 29-36.