Sale 1:

Sold for 41 keys

http://www.tf2outpost.com/trade/28812955 - B/O of 42

Seller: https://backpack.tf/profiles/76561198071869486#!/compare/1493164800/1493337600

Buyer: https://backpack.tf/profiles/76561198071012049#!/compare/1493251200/1493337600

X = 41

Sale 2: (Dupe)

Sold for 40 keys

Seller: https://backpack.tf/profiles/76561198045207853#!/compare/1496620800/1497312000

Buyer: https://backpack.tf/profiles/76561198047717809#!/compare/1496707200/1497312000

X = 40

Sale 3:

Sold for 37 keys

No outpost

Seller: https://backpack.tf/profiles/76561198061665716#!/compare/1497657600/1497744000

Buyer: https://backpack.tf/profiles/76561198045207853#!/compare/1497484800/1497830400

Keys were then spent on the Aussie RL

X = 37

Sale 4:

Sold with 8 keys for Haunted Ghosts Liquidator’s Lid

Seller: https://backpack.tf/profiles/76561198138718997#!/compare/1495929600/1496016000

Buyer: https://backpack.tf/profiles/76561198126190606#!/compare/1495670400/1496016000

Sellers:

42 keys:

http://i.imgur.com/z3pPGc7.png – Duped, definitely re-listed

http://i.imgur.com/9FQMQjT.png - Clean, definitely re-listed

Sales at:

37

40 (Duped)

41

The Dupe sale here probably means that the next suggestion can be based off of dupe sales, if I understand that rule correctly.

Going with 37-41.