The primary issue with trading outside of the US would be conncectivity. The speed of your connection is everything. I would think that you would be put at a severe disadvantage if you where trying to hit bids or lift offers once a stock is on the move. Especially, if it's from a 2nd world country with a telecom network that may be inferior to the good ole USA. Whether it be thickly traded like IBM or thin little issues.
But, this I don't know for certain.
I trade listed stocks in NYC thru REDI and don't have a problem. But, if I where to be offshore this may be an issue traveling thru the web, backbones and what have you.
Has anyone out there dealt with this and if so, what solution did they arrive at ???