Hi, please take a second look at the Guild Wars 2 forum post from Gaile Gray that you refer to as she has updated the thread with more information:
https://forum-en.guildwars2.com/forum/support/account/Item-Shop-US/page/1#post63197
Gaile gray:
I should clarify: A display error will be corrected. The fact that you’re seeing the gem Store with a currency different from your own may point to a fraudulent-acquired code. I think you are wise to want to check that out.
Also consider that EU customers who purchased GW2 from Zavvi / Play / NCsoft were able to set up an EU billing address during the open beta for the in game cash store. Only EU customers that purchased from CJS keys were unable to do this, and their billing options were limited to US region countries only.
I have opened a new thread on the Guild Wars 2 forum asking specifically about CJS keys here, you may be interested in looking into it:
https://forum-en.guildwars2.com/for...-I-bought-is-not-legitimate/page/1#post122868
Arenanet have specifically stated that there are seperate US and EU regions for Guild Wars 2 keys, not just a single region as CJS keys are falsely claiming:
http://www.guildwarsinsider.com/arenanet-guild-wars-2-client-region-locked/
As already mentioned, I have specifically asked on the Guild Wars 2 forum in a new thread for them to look into these issues.
If Arenanet can confirm that the keys you sold were valid EU region keys provided by NCsoft / Arenanet that is completely fine. However if Arenanet state that they did not provide you with these keys, and that they are not valid EU region keys, then you will have to refund them all.
I think it would be in CJS key's best interests to also contact Arenanet / NCsoft directly to try and get this issue resolved quicker, if the keys are legitimate EU region keys provided to you by Arenanet / NCsoft, then you have nothing to lose other than gaining confirmation of this directly from them.