lol.. he didnt told you his Hearthstone is not a key but a battle.net account with Hearthstone attached with it. I try to trade him too before with my tf2 keys. But i confused what his trading and i ask but he told me he says "I actually bought it myself, it's on a new battle.net account which I have set up with a new email I enjoyed it for a few days, and now want someone else to enjoy it :P, or if you want to try it out for a few days and pass it along, you can do that too"
Yeah he said it was a fresh battlenet account so I could change email and password.
I trusted him because he had a lot of rep on his steam profile but well, some people just dont mind losing their reputation for a game :/
oh i see he told you, well its a risk to deal with account. You can get a HS key anywhere or wait next month for Open Beta of it.
Add me on steam to discuss and trade