Who would want to trade GW2 gold for Guild Wars gold?
But, if you could find such a person, I can't see how Anet would stop you (although, just like trading different game currancy, no way to prevent yourself from being scammed.)
Unfortunately I'm afraid there's going to be lots of spam with people wanting to trade GW accounts for GW2 accounts and vice versa depending on how many people are dissapointed in GW2 or just want a way to buy a fully stocked GW account.