Using last 30 days.

Sale 1:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198400327203#!/compare/1522368000/1522454400

Buyer: https://backpack.tf/profiles/76561198310436331#!/compare/1518825600/1522454400

X = 7

Sale 2:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1523232000/1523318400

Buyer: https://backpack.tf/profiles/76561198429250783#!/compare/1523145600/1523318400

Bot.tf confirms: https://i.imgur.com/RAlQIRR.png

X = 6

Sale 3:

Sold for 5 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198311820950#!/compare/1522454400/1522540800

Buyer: https://backpack.tf/profiles/76561198136516477#!/compare/1522540800/1522713600

X = 5.5

Sale 4:

Sold for 5 keys + 10 ref?

Seller: https://backpack.tf/profiles/76561198311820950#!/compare/1522108800/1522454400

Buyer: https://backpack.tf/profiles/76561198200865258#!/compare/1522454400/1522540800

Same guy as the other 5 key + 10 ref sale, can guarantee that it sold for this amount.

X = 5.5

Sale 5:

Sold for 4 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1522368000/1522454400

Buyer: https://backpack.tf/profiles/76561198135443720#!/compare/1521590400/1522627200

X = 4.5

2 more bot.tf sales at ~5 keys, as well as a bot purchase at ~5 keys:

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

Could not find these sales.

Sellers:

6 keys:

https://i.imgur.com/55s5HzD.png

https://backpack.tf/item/6395055701

https://backpack.tf/item/2719229392

8 keys:

https://i.imgur.com/9vgG3Pg.png

https://backpack.tf/item/6632344017

Sales at:

4.5

5 (3x)

5.5 (2x)

6

7

Sale at 4.5, without rounding, ends up being lower than the highest quickbuyer: https://i.imgur.com/cnro0xU.png excluding 4.5.

7 looks high given the seller at 6 keys, and was also a mannco.trade bot sale for a scammer: https://backpack.tf/item/4396605557 excluding 7.

Going with 5-6.