It doesn't work that way, even in theory. In ideal conditions, you won't get anything and will be right back at square one. In practice, though, you have to pay trading commissions, at fist when you buy and then when you sell, so you will end up with less than when you start. The only way the scheme would work out if you manage to trigger either panic selling by shorting or panic buying by going long. But it is not a trivial task even if you have plenty of dough.