Fine Red Mist

You can search convert x.xx Keys/usd on backpack.tf and it converts it for you.

This was something that confused me when I started trading too. Not every weapon or item has strangifier, generally the only way to get a strange variant is by opening a crate/case. These are the weapons and items that have strangifier: https://wiki.teamfortress.com/wiki/Template:Strangifiers_table