sale 2 was almost certainly 2 keys + 7 ref. I don't have proof of it anymore unfortunately, but I was watching this item a while ago. The buyer had a buy order at 2 keys + 7 ref for any effect. They also bought another one for 2 keys + 7 ref

https://backpack.tf/profiles/76561198243929698#!/compare/1708128000/1708214400

https://backpack.tf/profiles/76561199088494686#!/compare/1708128000/1708214400

https://backpack.tf/item/14132810167

A couple other sales from the same history (some may be outdated now):

https://backpack.tf/profiles/76561198868211413#!/compare/1701475200/1707523200

https://backpack.tf/profiles/76561199581842147#!/compare/1707436800/1707523200

https://backpack.tf/item/13757279187

Untraceable, probably a backpack sale

https://backpack.tf/profiles/76561199581842147#!/compare/1707436800/1707523200

https://backpack.tf/profiles/76561199564942025#!/compare/1707436800/1707523200

https://backpack.tf/item/13757279187

Untraceable

https://backpack.tf/profiles/76561199088494686#!/compare/1708214400/1708300800

https://backpack.tf/profiles/76561197963995721#!/compare/1707868800/1709078400

https://backpack.tf/item/14132810167

SCM. Too many sales there to determine which one it was.

https://backpack.tf/profiles/76561198119130904#!/compare/1714003200/1714867200

https://backpack.tf/profiles/76561198061836787#!/compare/1713916800/1714003200

https://backpack.tf/item/14132810167

Probably a backpack buy

Marketplace sale at $5.55

https://marketplace.tf/items/tf2/31236;5;u3003

Can't find history

https://marketplace.tf/items/tf2/5021;6 - keys were $1.63 at the time

5.55/1.63 = 3.4

This is pretty high compared to the seller, so it would be best to exclude it.

With a current seller at ~2.5 keys, and two sales lower than 3, I'd probably try for 2.1-3 keys here.