Usually an admin will fix this if the person refuses to trade back. Give them a day or two to respond and if they don't, send a (polite) message to an admin.
Nope, admins will do nothing in such a case. It is your own fault if you ask for the wrong items.
Check the marketplace rules:
http://forum.melee.org/announcements/new-market-rule/Something similar happened to me yesterday, you just have to be more careful (thanks again Fin for your help
). Try to talk again to the other person, that's all you can do. Perhaps offer him some more gold on top.
Good luck getting your item back.