Sale 1:

Sold for Orbiting Fire Prancer's Pride + Steaming Doctor's Sack (Mini at 10 below)

http://www.tf2outpost.com/trade/29501083 - B/O of 18

Seller: https://backpack.tf/profiles/76561198077251918#!/compare/1514764800/1514937600

Buyer: https://backpack.tf/profiles/76561198398063696#!/compare/1514764800/1514937600

Easily caps.

X = 18 (Capped by B/O)

Sale 2:

Sold for Stormy Storm Swagman's Swatter (16.5)

http://www.tf2outpost.com/trade/29489705 - B/O of 16.5

Seller: https://backpack.tf/profiles/76561198100870346#!/compare/1514419200/1514592000

Buyer: https://backpack.tf/profiles/76561198307215279#!/compare/1514505600/1514592000

Gonna round down due to the B/O.

X = 16

Sale 3:

Sold for 15 keys in ~20 days

Seller: https://backpack.tf/profiles/76561198337734785#!/compare/1513987200/1514073600

Buyer: https://backpack.tf/profiles/76561198100870346#!/compare/1512950400/1514073600

X = 15

Sale 4:

Sold for 15 keys in ~2 days

Seller: https://backpack.tf/profiles/76561198398063696#!/compare/1514764800/1514937600

Buyer: https://backpack.tf/profiles/76561198129204745#!/compare/1514851200/1514937600

X = 15

Sale 5:

Sold for Cloud 9 Stout Shako

No outpost

Seller: https://backpack.tf/profiles/76561198129204745#!/compare/1515024000/1515110400

Buyer: https://backpack.tf/profiles/76561198370217844#!/compare/1515024000/1515110400

Given that the seller immediately quicksold the shako for a little more than 15 keys, it would probably be safe to assume that the B/O would've been around that much. Either way, Given the seller at 17 on the Shako, it may be better to update the shako with this sale instead.

Sale 6:

Sold with a key in items for Searing Plasma Toy Soldier

Seller: https://backpack.tf/profiles/76561198109871970#!/compare/1514764800/1514851200

Buyer: https://backpack.tf/profiles/76561198077251918#!/compare/1514678400/1514764800

Can probably be used on the Toy Tailor.

Steaming Sack Mini:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1515888000/1515974400

Buyer: https://backpack.tf/profiles/76561198132053816#!/compare/1515888000/1515974400

X = 10

Sellers:

25 keys:

https://i.imgur.com/6bswWN9.png - 3 days

Sales at:

15 (2x)

16

18

Going with 15-18.