"For marketplace.tf sales we dont consider the 10% commision that sellers have to pay" -> No need for the low-end here.

You also need to find histories of items for MP sales (not valid if duped for example)

You'll need to find more sales if there are any (by browsing past suggestions for histories for example)

https://mannco.store/item/17615 - Sold for $31.17 (18 Keys)

https://mannco.store/item/1 - Keys were $1.69 each

https://backpack.tf/item/5088950589 - This might've been the Marketplace sale's history? I can't find it

https://backpack.tf/item/7789998994 - The sales here are untraceable

As Apothic said, you don't factor the tax into the calculation, so this should be resuggested at 18 - 20 Keys.