Can you tell me if this is still happening for you? I am able to purchase from shop search fine. If you are still having this issue, please let me know your browser, operating system and if you have any browser addons.

I am thinking it might be one of the addons since I am on Chrome as well with no problem. You may want to disable each, one at a time and see if it is one of those.

I also get this on Chrome. I just tried updated Chrome as well and it didn't help (Version 45.0.2454.99 m).

: Could you take a screenshot of the error message? I am not able to recreate it.

: Also - can you give a recent item you've tried so that I can try?

: Hm, very strange. I was able to purchase it without an issue... But I just pushed a potential fix live to the main site. Could you try to purchase another item in 3-5 minutes?

Another user who was having this issue said it was resolved with the new shops. Are you still getting the error?