Seller:

20 keys, 19 days

Sales:

1.

https://marketplace.tf/items/30779;5;u36

~12.5

X1 = 12-13

---

2.

B\O of 20

http://www.tf2outpost.com/user/76561198042285872

Sold for Steaming Cannonball (17 keys, 3 months)

https://backpack.tf/profiles/76561198042285872#!/compare/1503964800/1504396800

Bot at 16, one week

Sold here for more than 17 in items

https://backpack.tf/profiles/76561198216505873#!/compare/1505865600/1505952000

Taking current price.

X2 = 17

---

3.

Sold for Max's Head

17.7 - 18 at that time, 18-18.5 now. Taking 18.

https://backpack.tf/profiles/76561198131579314#!/compare/1503705600/1504483200

X3 = 18.

---

4.

Idk.

He sold another hat to him for keys

https://backpack.tf/profiles/76561198250722514#!/compare/1502323200/1502582400

https://backpack.tf/item/4126041853

https://backpack.tf/item/5373404003

And here you can see 15 keys from him

https://backpack.tf/profiles/76561198048851745#!/compare/1502496000/1502668800

Probably sold his Pofessor's Peculiarity and bought Dayjogger

X4 = 15

---------------------------------------------------------------------------------------------------

Sale #1 is coutered by the buy order

Sale #4 is doubtfull

Taking 17-18.