I'm not sure that's right Spacecat,although I stand to be corrected, I'm pretty sure you can effectively "sell" shares that you don't have in the hope that they go down rather than up.
I think it's called "short selling".
You're right about the tax thing though as well as the cost of buying and selling. Although you could also view
betfair's commision as a form of "tax". The difference between
betfairs "tax" and normal tax though is that the more you win, the lower the "tax" rate is
