Exedently bought, Gift of Baelfire
I’m very sorry for your loss. You could file a support ticket at http://support.guildwars2.com/ (ask a question) to make this request.
However, following this forum for a long time the answer will likely be negative. It is not Arenanet’s policy to refund things bought with ingame currency, specially when it is your own mistake and not theirs. They are even claiming that they dont have the mod tools to do so. Wether this is true claim or not remains point of discussion, bottom line will be that they won’t do this.
Sorry to bring you this bad news.
Arise, opressed of Tyria!
Once a friend bought full CoF armor set for medium when he wanted it for light. He sent a ticket, they said, unfortunatelly there is nothing that we can do about it. That is why every time you buy something from these vendors it asks you if you want it before it purchases.. but still happens :/
Time to make Volcanus/Incinerator!
Your mistake is not a game bug though so this is in the wrong section.