This question already has an answer here:
Lately there have been quite a bit selling/buying rate differences between different BTC exchanges. Now some of this can be explained by the fact that different exchanges operate based on different real world currencies and there is nothing to it, as the currency switches will diminish all or most differences between BTC rates.
However, since some of the exchanges base their operations on the same real world currency and yet still have noticeable difference between exchange rates, this opens up an interesting possibility for speculation. Note that I haven't done this on a big scale and do not recommend that anyone short on cash does any actual experimentation with this.
So here is a thought and the question. What prevents from X doing the following: first buy BTC from exchange A with real world currency M, then seconds after BTC has settled to X's wallet, he/she then proceeds to put BTCs on sale on echange B for the same real world currency BTC was bought with from exchange A; now assuming, for example, that exchange A's selling rate is N and B's buying rate is N+20USD, this sounds as real as any day trading.
Now what I wonder is that is this really exploitable in money-making sense or do the delays with transactions or some other mechanisms prevent anyone benefiting from different exchange rates on different exchanges?