Sale 1:

Sold for Abduction Head Full of Hot Air (22.5)

No outpost

Seller: https://backpack.tf/profiles/76561198275530188#!/compare/1495065600/1495238400

Buyer: https://backpack.tf/profiles/76561198118921483#!/compare/1495065600/1495238400

X = 23

Sale 2:

Sold with a key for Circling Heart Backbiter’s Billycock (22.5)

http://www.tf2outpost.com/trade/28881012 - B/O of 19

Seller: https://backpack.tf/profiles/76561198069813252#!/compare/1494806400/1494892800

Buyer: https://backpack.tf/profiles/76561198253570951#!/compare/1494806400/1494892800

X = 19 (Capped by B/O)

Sale 3:

Sold for Hellish Inferno Spent Well Spirits (18.5) + ~0.5 keys in metal/items

No outpost

Seller: https://backpack.tf/profiles/76561198321427778#!/compare/1495497600/1496534400

Buyer: https://backpack.tf/profiles/76561198065256484#!/compare/1496448000/1496534400

X = 19

Sale 4:

Sold for Aces High Baker Boy (18) + >1.5 keys in items

http://www.tf2outpost.com/trade/29191271 - B/O of 18

Seller: https://backpack.tf/profiles/76561198118921483#!/compare/1495238400/1495324800

Buyer: https://backpack.tf/profiles/76561198262787556#!/compare/1495238400/1495324800

X = 18 (Capped by B/O)

Sale 5:

Sold for 18 keys

http://www.tf2outpost.com/trade/29194210 - B/O of 18

Seller: https://backpack.tf/profiles/76561198124728984#!/compare/1496361600/1496534400

Buyer: https://backpack.tf/profiles/76561198040726479#!/compare/1495843200/1496620800

X = 18

Sale 6:

Sold in ~5 days for 14 keys + ~2 keys in metal/items

http://www.tf2outpost.com/trade/29184610 - B/O of 18

Seller: https://backpack.tf/profiles/76561198150003387#!/compare/1495324800/1495411200

Buyer: https://backpack.tf/profiles/76561198124728984#!/compare/1495324800/1495411200

X = 16

Sale 7:

Bulk for Hat Protector Phononaut + keys

Seller: https://backpack.tf/profiles/76561198211995121#!/compare/1491868800/1492041600

Buyer: https://backpack.tf/profiles/76561198069813252#!/compare/1491955200/1492041600

Sale 8:

Bulk for Slumber Bitey

Seller: https://backpack.tf/profiles/76561198069225485#!/compare/1494547200/1494633600

Buyer: https://backpack.tf/profiles/76561198041100478#!/compare/1494201600/1494633600

Sellers:

18 keys:

http://www.tf2outpost.com/trade/29210364 - 2 weeks

https://backpack.tf/u/76561198040726479

20 keys:

http://www.tf2outpost.com/trade/29201731 - 3 weeks

https://backpack.tf/u/76561198053747026

Other seller on outpost already sold.

Sales at:

16

18 (2x)

19 (2x)

23

23 looks like an outlier, based on other sales and current sellers. I can also guarantee that the sale would be much lower if the B/O was present. Excluding 23.

16 sold in a somewhat shortish amount of time, but long enough that I would normally include in the range in most circumstances. However, given that sales are much more common in the 18 to 19 region, and in contrast with how quickly the sale at 16 occurred, I will be excluding 16 here.

Going with 18-19.