Steam Trading Error – “Your Inventory Is Not Available At This Time”

This means that - for whatever reason - the built-in browser handling these pages somehow lost the data identifying the current session (which is essentially just some semi-random value to identify the computers allowed to actually see the data and modify it; i.e. those that are allowed to complete the trade).

There are several reasons why this could happen:

• The whole trade session might have been open simply too long. For example, you might have opened the trade and then didn't try to complete it "in time". I don't know how long that time is, but I'd assume at least a few minutes (unlikely more than an hour).
• Some server handling the request(s) dropped that Information. This could be a local proxy or just some server at Valve acting up.
• Your Internet connection dropped for a few seconds while you tried to perform the trade. In this case you might have been assigned a new IP by your ISP, which most likely would invalidate the current session for security purposes.
• If you're using some kind of firewall or filter software modifying the content of web pages (to "protect" you or remove ads), this might be the reason as well (filtering this legitimate data intentionally or by accident). Try to add the Steam client to its exception list.

This is a common issue with no real solution. The following has worked for a lot of people:

1. Open Internet Explorer
2. Internet options ( Top right is a Gear Icon)
3. Go to Privacy tab
4. Advance
5. check " Override automatic cookie handling"
6. Select Accept for both First-Party Cookies & Thrid-Party Cookies
7. Check under " Always allow session cookies"
8. Ok > Apply > Ok
9. Close Internet Explorer and Restart Steam

However, this has not worked for me; I still have this problem sporadically. The solution for me has been to skip Steam entirely by logging into the Steam website using any browser, and selling my items from the market page.