Using last month.

Sale 1:

Sold for 4 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1552089600/1552176000

Buyer: https://backpack.tf/profiles/76561198196894698#!/compare/1552089600/1552176000

https://backpack.tf/item/7450387536

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 4

Sale 2:

Sold to a bot for 3 keys + 34 ref

Seller: https://backpack.tf/profiles/76561198196894698#!/compare/1552089600/1552176000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1552089600/1552176000

https://backpack.tf/item/7450387536

https://i.imgur.com/mULDhZn.png - bot.tf confirms

Note: this sale was a tradeback on the same day as the purchase above, hence why the links are the same.

X = 4

Sale 3:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198382654640#!/compare/1551484800/1551571200

Buyer: https://backpack.tf/profiles/76561198192494651#!/compare/1551571200/1551657600

https://backpack.tf/item/3392666016

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 4

Sale 4:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1551312000/1551398400

Buyer: https://backpack.tf/profiles/76561198877320307#!/compare/1551312000/1551398400

https://backpack.tf/item/3341488125

X = 4

Sale 5:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1550188800/1550275200

Buyer: https://backpack.tf/profiles/76561198246122716#!/compare/1550016000/1550275200

https://backpack.tf/item/2936582816

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 4

Sale 6:

Sold with 10 ref for 4 keys

Seller: https://backpack.tf/profiles/76561198413913898#!/compare/1551657600/1551744000

Buyer: https://backpack.tf/profiles/76561198271293393#!/compare/1551571200/1551744000

https://backpack.tf/item/2861578231

X = 4

Sale 7:

Sold for 3 keys + 30 ref

Seller: https://backpack.tf/profiles/76561198367292038#!/compare/1550361600/1550448000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1550448000/1550534400

https://backpack.tf/item/3341488125

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 3.5

Sale 8:

Sold to a bot for 3 keys + 28 ref

Seller: https://backpack.tf/profiles/76561198246122716#!/compare/1550620800/1550707200

Buyer: https://backpack.tf/profiles/76561198335791307#!/compare/1550534400/1550620800

https://backpack.tf/item/2936582816

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 3.5

Sale 9:

Sold to a bot for 3 keys + 24 ref

Seller: https://backpack.tf/profiles/76561198282962719#!/compare/1551312000/1551398400

Buyer: https://backpack.tf/profiles/76561198340634356#!/compare/1551312000/1551398400

https://backpack.tf/item/2705432700

https://i.imgur.com/mULDhZn.png - bot.tf confirms

X = 3.5

Sale 10:

Sold for 3 keys

Seller: https://backpack.tf/profiles/76561198132450305#!/compare/1552176000/1552262400

Buyer: https://backpack.tf/profiles/76561198363395191#!/compare/1552176000/1552262400

https://backpack.tf/item/3288456072 (everything else in this history is gambling-related)

X = 3

Marketplace Sales:

https://marketplace.tf/items/1116;5;u3003

https://manic.tf/keyprice/?at=2019-03-02 - $6 / $1.79 = 3.35 keys (2x)

X = 3.5 (2x)

Other Sales:

https://backpack.tf/item/7349362458 - scammed

https://backpack.tf/item/3178070902 - scammers

https://backpack.tf/item/4111498728 - scrap

https://backpack.tf/item/2823189390 - tradeit

https://backpack.tf/item/7427485026 (#1)

https://backpack.tf/item/6990647733 (#2)

https://backpack.tf/item/3341488125 (#3 & #4)

https://backpack.tf/item/3035434264 (#5-7)

https://backpack.tf/item/2936582816 (#8)

https://backpack.tf/item/2705432700 (#9 & #10)

#1

Untraceable

https://backpack.tf/profiles/76561198102377117#!/compare/1550793600/1550880000

https://backpack.tf/profiles/76561198340634356#!/compare/1550793600/1550880000

https://i.imgur.com/mULDhZn.png - Labelled as 3 keys + 24 ref

The buyer re-sold to an unscanned backpack, making the next sale untraceable too. That sale was labelled as 4 keys on bot.tf.

#2

Untraceable

https://backpack.tf/profiles/76561198130439466#!/compare/1551571200/1551657600

https://backpack.tf/profiles/76561198058025383#!/compare/1550620800/1551744000

#3

Untraceable

https://backpack.tf/profiles/76561198192494651#!/compare/1551657600/1551744000

https://backpack.tf/profiles/76561198340634356#!/compare/1551571200/1551657600

https://i.imgur.com/mULDhZn.png - labelled as 3 keys + 32 ref

#4

Untraceable

https://backpack.tf/profiles/76561198877320307#!/compare/1551312000/1551398400

https://backpack.tf/profiles/76561198073522694#!/compare/1551312000/1551398400

#5

Sold for Pro. KS Shotgun

https://backpack.tf/profiles/76561198272560640#!/compare/1551312000/1551657600

https://backpack.tf/profiles/76561198146579306#!/compare/1551571200/1551657600

Too high, excluding.

#6

Untraceable

https://backpack.tf/profiles/76561198146579306#!/compare/1551657600/1551744000

https://backpack.tf/profiles/76561198081338863#!/compare/1551657600/1551744000

#7

Untraceable

https://backpack.tf/profiles/76561198081338863#!/compare/1551657600/1551744000

https://backpack.tf/profiles/76561198340634356#!/compare/1551657600/1551744000

https://i.imgur.com/mULDhZn.png - Labelled as 3 keys + 32 ref

#8

Untraceable

https://backpack.tf/profiles/76561198133534437#!/compare/1549843200/1550188800

https://backpack.tf/profiles/76561198305676157#!/compare/1550102400/1550188800

#9

Untraceable

https://backpack.tf/profiles/76561198340634356#!/compare/1551312000/1551398400

https://backpack.tf/profiles/76561198877320307#!/compare/1551312000/1551398400

https://i.imgur.com/mULDhZn.png - labelled as 4 keys

#10

Untraceable

https://backpack.tf/profiles/76561198877320307#!/compare/1551312000/1551398400

https://backpack.tf/profiles/76561198073522694#!/compare/1551312000/1551398400

Sellers:

https://i.imgur.com/x2Ow56h.png

https://i.imgur.com/HWTdgYp.png

https://backpack.tf/item/7450387536 - ~4.05 keys

https://backpack.tf/item/2936582816 - ~4.05 keys

https://backpack.tf/item/3288456072 - ~4.05 keys

https://backpack.tf/item/3392666016 - ~4.1 keys

https://backpack.tf/item/3626154732 - 5 keys

https://backpack.tf/item/6516500321 - 9 keys

https://backpack.tf/item/2693404012 - 9 keys

Sales at:

3

3.5 (5x)

4 (6x)

3 is countered by 1 valid buyer. Pauky has been known to be an unreliable buyer. https://i.imgur.com/bkqJq7U.png

Going with 3.5-4.