Steam Trading Error - "Your Inventory Is Not Available At This Time"

I experienced exactly the same thing some time ago.

I have no exact explanation, but my bet is that it's something with steam servers. Either maintenance or some outage. So the only solution would be to just wait for server to go back to normal, unfortunately.

Writing to support might help(or at least give a bit more info) but they do not answer questions quickly and the problem would go away when they would answer, I guess...

Ok, for anyone with the query, a while ago Steam replied with this:

Please exit Steam and go to the folder called C:\Program Files\Steam\ (this is the default location for a Steam installation - if you set a different installation directory, you will need to browse to it).

Delete all of the files in this folder except:

The \Steamapps\ folder and Steam.exe (this file is listed as an application and features the black and white Steam logo)

Restart your computer.

Then, launch Steam.exe from within the Steam installation folder, and not from a pre-existing shortcut.

With Steam running, re-test the original issue.

Note: This process will not affect your currently installed games.

This did appear to fix it for me although it does still happen on occasion. (not very often though)