3 months+ with this bug

Same problem

You hit an API quota limit. This quota limit is IPv4 based.

First, try clearing the web cache of the Steam client to see if it is just a cached page issue.

If that doesn't help, restart your router, this should give you a new IPv4 address.

If restarting your router doesn't help, you'll have to wait it out.

If you waited several days to weeks, your only recourse is getting a public IPv4 address. It doesn't matter whether this one is static or dynamic. Ask your ISP on how to obtain one.

Reason:
IPv4 addresses are not unique anymore. As there are more humans (8 billion) than IPv4 addresses (4.2 billion), ISPs have chosen to route users together on an identical IPv4 address. This technique is known as CGNAT.[en.wikipedia.org]
For Valve's inventory and community market API this bundled together group of users using CGNAT or Dual Stack Lite looks like a single user.

Workaround if you are behind Dual Stack Lite:
If you are behind Dual Stack Lite (shared IPv4 address, dedicated IPv6 prefix), temporarily disable IPv4 on your system after you made sure IPv6 support is enabled in your router to force Steam to use IPv6. Undo this step after you finished using the market or inventory (you will have to redo it every time you use either). Do not use this as a permanent solution because not every service on the internet did implement IPv6 support yet.

If you already have a public also known as dedicated IPv4 address, you should stop spamming inventory actions.