Hello,
I was fortunate to win a beta key for hearthstone through the danish Eurogamer site and have attached a screenshot proving that:
I believe it is an EU-key, but both EU and US keys work for EU and US people.
I'm selling it for 150$ (or highest bidder)
I haven't done this before (which is also why this account is new), but I imagine we figure something out over skype (PM your offer) and you will receive the key after payment has gone through.
I'm not sure what other proof I could provide of the key's validity, but feel free to request whatever would make you comfortable about the trade.
A middleman would also be acceptable.