Sale 1:

Sold for Stormy Storm Desert Marauder (Suggestion at 17) + 7 keys (other keys can be seen in the buyer’s compare link)

No outpost

Seller: https://backpack.tf/profiles/76561198045388088#!/compare/1502323200/1502582400

Buyer: https://backpack.tf/profiles/76561198079504588#!/compare/1502496000/1502582400

X = 24

Sale 2:

Sold for 23 keys

http://www.tf2outpost.com/trade/29371047 - B/O of 25

Seller: https://backpack.tf/profiles/76561198043936328#!/compare/1506816000/1506902400

Buyer: https://backpack.tf/profiles/76561198341453773#!/compare/1506816000/1506902400

X = 23

Sale 3:

Sold for 21 keys

https://i.imgur.com/yFzKIhV.png - Re-selling, after purchasing again, for 21 keys. Assuming same B/O.

Seller: https://backpack.tf/profiles/76561198304138933#!/compare/1506556800/1506643200

Buyer: https://backpack.tf/profiles/76561198147318341#!/compare/1506556800/1506643200

X = 21

Sale 4:

Quicksold for 20 keys

No outpost

Seller: https://backpack.tf/profiles/76561198119125919#!/compare/1501891200/1501977600

Buyer: https://backpack.tf/profiles/76561198045388088#!/compare/1501891200/1501977600

X = 20

Sale 5:

Bulk for things

Seller: https://backpack.tf/profiles/76561198125791160#!/compare/1502150400/1502236800

Buyer: https://backpack.tf/profiles/76561198008113369#!/compare/1502064000/1502236800

Stormy Marauder:

https://backpack.tf/suggestion/59e8cc72cf6c754364519002

Sellers:

21 keys:

https://i.imgur.com/YDDcYqQ.png - 17 days

https://i.imgur.com/piqvq38.png - 12 days

24 keys:

https://i.imgur.com/86yBexs.png - 15 days

Sales at:

20

21

23

24

20 quicksold, excluding 20.

Going with 21-24.