Sale 1:

Sold for Logo Coupe (Suggestion at 11) + Orbiting Fire Counterfiet Billycock (10)

http://www.tf2outpost.com/trade/29380118 - No visible B/O

Seller: https://backpack.tf/profiles/76561198150783829#!/compare/1506643200/1506729600

Buyer: https://backpack.tf/profiles/76561198254469336#!/compare/1506643200/1506729600

X = 21

Sale 2:

Sold for DBD Rubber Glove (Suggestion at 18)

No outpost

Seller: https://backpack.tf/profiles/76561198254469336#!/compare/1506729600/1506816000

Buyer: https://backpack.tf/profiles/76561198137566942#!/compare/1506729600/1506816000

X = 18

Sale 3:

Sold for Disco Beat Down Hive Minder (Suggestion at 16) + 1 key

http://www.tf2outpost.com/trade/29440898 - B/O of 21

Seller: https://backpack.tf/profiles/76561198255903931#!/compare/1510531200/1510617600

Buyer: https://backpack.tf/profiles/76561198098096360#!/compare/1510531200/1510617600

X = 17

Sale 4:

Sold for Miami Nights Bullet Buzz + cheap items

No outpost

Seller: https://backpack.tf/profiles/76561198098096360#!/compare/1510531200/1510617600

Buyer: https://backpack.tf/profiles/76561198042469379#!/compare/1510444800/1510617600

No sales on the Buzz.

DBD Glove:

https://backpack.tf/suggestion/5a1d510544325a673d77a59f

DBD Hive Minder:

https://backpack.tf/suggestion/5a1d6cd7cf6c7549841a9456

Sellers:

20 keys:

https://i.imgur.com/2uE5sLV.png - 15 days

https://i.imgur.com/xzI8BHI.png - 3 days

21 keys:

https://i.imgur.com/FR0R03y.png - 2 months

https://i.imgur.com/Ti91pGq.png - 9 days

Sales at:

17

18

21

Sellers at 20-21, especially the 2-month old seller at 21, show that 21 is too high. Excluding 21.

Going with 17-18.