The Guild bank is fairly safe as long as you don't drop it in and then just pull out on the other account, let it sit there and pull the gold/items out over time. As for the ah or trade you can't sell some random cheap item and buy it for 10k, you need to keep the value to price ratio within say 150%. Also if you do it enough they may catch on as that was a popular way to buy/sell gold in the past.