Hey guys, Bitcoin is over $16,000 each on Coinbase/GDAX. Yay. Sorry I was asleep when it hit $15,000 [Btw, blame the (S.) Koreans for this huge rally as they are buying bitcoins there like there's no tomorrow at the Bithumb exchange. They were buying bitcoins yesterday at over $17K each, today they are paying over $18K and that exchange has the highest volume worldwide]
Anybody with a gdax account should open an account on bitstamp and do the arb. Unfortunately trying to open an account at gdax is currently producing an error message.
I don't know the answer. Bitcoins at one exchange are different from another exchange and they don't share the same supply so if the Koreans are buying and paying whatever price on that exchange, there's just not enough supply at lower prices so they keep paying higher and higher prices as Bithumb sources from other exchanges and pushinghigher prices from say GDAX. That's my very basic guess at the situation.