So it's been over a year since I first came across this problem and I'm still experiencing it. In Chrome and only Chrome, every attempt to buy something from a user shop within the search will give me an "Item doesn't exist!" message.

Operating Systems: Windows 7, Lubuntu Linux Browsers: Google Chrome, Chromium (respectively) Extensions: Tumblr Savior, Avast site checker, Ghostery

Happens in both Windows and Linux, and regardless as to whether the extensions are on or off. Firefox, however, even while running Adblocker, has no problems on either OS, but I'm getting exceedingly tired of having to run two browsers at the same time just so I can do quests, especially with how sluggish Firefox is on my machine.

Just tested it (have been using Firefox on my new machine, put Chrome on an older machine just now), appears to have been, probably with the shop changes!

I had the same issue, when the shop search was changed for the new shops, it went away.