Full credits to Mindacos

Sales 1 & 2:

Sold for $31 and #32 on Marketplace(~16 Keys using 1.95 value, and ~16 Keys using 1.98 value)

https://marketplace.tf/items/388;5;u33;strange

X = 16 (2x)

Sale 3:

Sold for an Australium Minigun(16 Keys)

https://backpack.tf/profiles/76561198290639639#!/compare/1507248000/1507334400

X = 16

Sale 4:

Sold for an Australium Flame Thrower(~14.5 Keys)

https://backpack.tf/profiles/76561198253835120#!/compare/1503964800/1504051200

X = 15

Sale 5:

Sold for 15 Keys

https://backpack.tf/profiles/76561198133540930#!/compare/1508025600/1508112000

X = 15

Sale 6:

Sold for 14 Keys

https://backpack.tf/profiles/76561197960833481#!/compare/1510617600/1510704000

X = 14

Sale 7:

Sold for 13 Keys

https://backpack.tf/profiles/76561198044943825#!/compare/1509753600/1509840000

http://www.tf2outpost.com/trade/29418596

X = 13

Sale 8:

Sold for 13 Keys

https://backpack.tf/profiles/76561198193760642#!/compare/1504656000/1504742400

X = 13

Sellers:

15 keys:

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

16 keys:

https://i.imgur.com/urafoZb.png - 4 months (2x)

17 keys:

https://i.imgur.com/UAOUPv5.png - 4 days

Sales at:

13 (2x)

14

15 (2x)

16 (3x)

4 month old seller at 16, though there are quite a few sales at 16, so I won’t exclude it.

Going with 13-16.