Invalid Cookie
In some cases, the Steam Community Market is unable to read your browser's cookie. In most cases, logging out of your Steam account and logging back in will resolve this. If this doesn't fix the issue then you may be running a program or browser add-on that is interfering with Steam's cookie (click here to learn more about this issue). In rare cases you may have to try another browser or delete the Steam cookie from your browser; however, this will likely result in a new device restriction (see the Newly Authorized Device section above).