Re-suggestion, thanks to K1NG for the extra info!

Sale 1:

https://backpack.tf/profiles/76561198068127299#!/compare/1528329600/1528416000

https://backpack.tf/profiles/76561198815852225#!/compare/1528329600/1528416000

Sold for MS Kick(20.5)

http://www.tf2outpost.com/trade/29677642

B/O of 17 keys

Capping at 17.

Sale 2:

https://backpack.tf/profiles/76561198815852225#!/compare/1528416000/1528588800

Use there.

Sale 3:

https://backpack.tf/profiles/76561197960984512#!/compare/1528416000/1528502400

https://backpack.tf/profiles/76561198439812057#!/compare/1528416000/1528502400

Sold for Aces Tree + ~ key in sweets

Tree Mini:

https://backpack.tf/profiles/76561198198332539#!/compare/1528156800/1528243200

Sold for 18 keys

Considering seller for 17, taking 17 - 18 keys. Average of 17.5

Total is 18.5 keys.

Sale 4:

https://backpack.tf/profiles/76561198121345519#!/compare/1528502400/1528588800

Sold for 14 keys

Sale 5:

https://backpack.tf/profiles/76561198344933095#!/compare/1530144000/1530230400

https://backpack.tf/profiles/76561198141636088#!/compare/1530057600/1530230400

Sold for 14 keys + 14 ref

Sale 6:

https://backpack.tf/profiles/76561198121345519#!/compare/1529107200/1529193600

https://backpack.tf/profiles/76561198126693824#!/compare/1529366400/1529452800

Untrace.

Sale 7:

https://backpack.tf/profiles/76561198126693824#!/compare/1529452800/1529539200

https://backpack.tf/profiles/76561198090278303#!/compare/1529366400/1529539200

Sold for Stormy Whoopee Cap + ~1 key in sweets

Unable to mini.

Sale 8:

https://backpack.tf/profiles/76561198090278303#!/compare/1529884800/1529971200

https://backpack.tf/profiles/76561198361723669#!/compare/1529193600/1529884800

Untrace.

K1NG'S Part:

The Aces Tree is one month unsold at ~15, and the OF Mountain cap is 3+ weeks unsold at 16 in classies, so this will be high:

https://backpack.tf/classifieds?item=A+Rather+Festive+Tree&quality=5&tradable=1&craftable=1&particle=59&australium=-1&killstreak_tier=0

https://backpack.tf/classifieds?item=Medic%27s%20Mountain%20Cap&quality=5&tradable=1&craftable=1&australium=-1&particle=33&killstreak_tier=0

There are also other recent sales worth checking

https://backpack.tf/item/1556510540

https://backpack.tf/item/439030509

I think 14-17 should be ok.

Going with 14-17 keys.