Using sales from April onwards.

Sale 1:

Sold for 4 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1525478400/1525564800

Buyer: https://backpack.tf/profiles/76561198118205224#!/compare/1525564800/1525651200

X = 4.5

Sale 2:

Sold to a quickbuyer for 4 keys

Seller: https://backpack.tf/profiles/76561198126835242#!/compare/1523232000/1523750400

Buyer: https://backpack.tf/profiles/76561198091502579#!/compare/1523232000/1523318400

X = 4

Sale 3:

Sold to a bot for 3 keys + 1 ref

Seller: https://backpack.tf/profiles/76561198148304461#!/compare/1519603200/1527033600

Buyer: https://backpack.tf/profiles/76561198312265776#!/compare/1526774400/1526860800

Bot.tf confirms: https://i.imgur.com/b91W29O.png

X = 3

Sellers:

4 keys:

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

~4.4 keys:

https://i.imgur.com/wk6UGwJ.png

https://backpack.tf/item/4438481706

https://backpack.tf/item/3927721442

~4.6 keys:

https://i.imgur.com/zzd4Wvn.png

https://backpack.tf/item/3919763593

7 keys:

https://i.imgur.com/hS9upfe.png

https://backpack.tf/item/5237740412

https://backpack.tf/item/4022283200

8 keys:

https://i.imgur.com/hMt9pGv.png

https://backpack.tf/item/6405372420

20 keys:

https://i.imgur.com/C34FgP7.png

https://backpack.tf/item/4037024141

Sales at:

3

4

4.5

3 might be a bit low, excluding 3.

Ignoring the buyer at 4+ keys, they’re most likely not actually buying.

Going with 4-4.5.