Really, I suggest you don't do tradings..
Trading isn't allowed, so Blizzard would ban your account then. Also, I don't know the Euro to USD ratio, but 800 euros does sound like a decent chunk of change.. But, if he is this serious, I say ask him, who's your lawyer? The lawyers/judge/juries will laugh at you because you're filing a lawsuit over a Video game, and you both broke the rules by trading accounts, so yeah...