Using last month.

Sale 1:

Sold for Holy Grail Victory Lap (Easily above 17)

http://www.tf2outpost.com/trade/29626009 - B/O of 17

Seller: https://backpack.tf/profiles/76561198072240925#!/compare/1524009600/1524268800

Buyer: https://backpack.tf/profiles/76561198251370744#!/compare/1524182400/1524268800

X = 17 (Capped by B/O)

Sale 2:

Sold for 16 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524960000/1525046400

https://backpack.tf/item/5075980411 - Buyer can’t yet be identified.

Bot.tf confirms: https://i.imgur.com/j5sphYw.png

X = 17

Sale 3:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198017911615#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198042798383#!/compare/1524614400/1524700800

X = 15

Sale 4:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198168821447#!/compare/1523750400/1524182400

Bot.tf confirms: https://i.imgur.com/j5sphYw.png

X = 15

Sale 5:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198069813252#!/compare/1523491200/1523577600

Buyer: https://backpack.tf/profiles/76561198074768807#!/compare/1523491200/1523577600

X = 14

Sale 6:

Bulk for Cloudy Moon Defiant Spartan

Seller: https://backpack.tf/profiles/76561198086973403#!/compare/1523145600/1523664000

Buyer: https://backpack.tf/profiles/76561198132226518#!/compare/1523491200/1523577600

Sellers:

19 keys:

https://i.imgur.com/VNiERkN.png

https://backpack.tf/item/4033464653

24 keys:

https://i.imgur.com/tShTwgP.png

https://backpack.tf/item/4267455999

25 keys:

https://i.imgur.com/PPrHNPJ.png

https://backpack.tf/item/3000838313

30 keys:

https://i.imgur.com/G9HAF2Q.png

https://backpack.tf/item/2698151703

32 keys:

http://www.tf2outpost.com/trade/29642576

Offers:

http://www.tf2outpost.com/trade/29436851

Sales at:

14

15 (2x)

17 (2x)

Going with 14-17.