Sale 1:

Sold with a key for 17 keys in items

http://www.tf2outpost.com/trade/29451649 - B/O of 15

Seller: https://backpack.tf/profiles/76561198129073740#!/compare/1511308800/1511395200

Buyer: https://backpack.tf/profiles/76561198104674110#!/compare/1511308800/1511395200

X = 15 (Capped by B/O)

Sale 2:

Sold with 3 keys + ~0.5 keys in items for Cloud 9 Nano (Suggestion at 15.5)

No outpost

Seller: https://backpack.tf/profiles/76561198254039787#!/compare/1509408000/1509580800

Buyer: https://backpack.tf/profiles/76561198154941386#!/compare/1509494400/1509580800

X = 12

Sale 3:

Bulk for Haunted Phantasm Jr. Cyborg Stunt Helmet

Seller: https://backpack.tf/profiles/76561198247572926#!/compare/1510444800/1511049600

Buyer: https://backpack.tf/profiles/76561198016070169#!/compare/1510790400/1511049600

Sale 4:

Sold for Eerie Orbiting Fire Coupe D’isaster

No outpost

Seller: https://backpack.tf/profiles/76561198127938353#!/compare/1511222400/1511308800

Buyer: https://backpack.tf/profiles/76561198206471180#!/compare/1510963200/1511136000

No mini available. What a D’isaster!

Sale 5:

Sold for Cyclone I See You

Seller: https://backpack.tf/profiles/76561198206471180#!/compare/1511136000/1511222400

Buyer: https://backpack.tf/profiles/76561198129073740#!/compare/1511136000/1511222400

Probably best to not use this sale.

Sellers:

~14.95 keys:

http://www.tf2outpost.com/trade/29451649 - 1 day

15 keys:

https://i.imgur.com/aZxWzoG.png - young

16 keys:

https://i.imgur.com/HCm7PSs.png - 6 days

https://i.imgur.com/0kp02WG.png - 4 days

https://i.imgur.com/3iH6f0s.png - 1 day

18 keys:

https://i.imgur.com/7Hhda4d.png - 24 days

19 keys:

https://i.imgur.com/gNj7H4C.png - 12 days

21 keys:

https://i.imgur.com/PQBGFWk.png - 3 months

https://i.imgur.com/AGVNeFR.png - 3 months

25 keys:

http://www.tf2outpost.com/trade/29402679 - 1 month

Sales at:

12

15

Going with 12-15.