I flipped the GE with at least 60m bank. Profits was 200-650k/hr (Large range due to volatile market)
In order to flip items, there are items to look out.
1) Items with low price, large buy/sell volumes
2) Mid range items with decent price, decent buy/sell volumes
3) High end ~ Rares, low buy/sell volumes
With a starting bank of 20m, highly suggested to do [1]. Or [2] if you know what you are doing.