I buy and sell stock through my bank. The various bank/broker/tax overheads, if I was to buy stock and immediately sell it at the same price, is roughly 3%. I am seeking to understand what exactly those overheads are, and how I could minimise them.
To take an extreme case, high frequency traders make orders on millisecond timeframes, a timeframe too small for stock to fluctuate more than 3%. So, necessarily, high frequency traders enjoy a smaller "buy/sell roundtrip" transaction overhead of less than 3%.
For specificity, let's pick the London Stock Exchange. What are the smallest transaction fees available there? How can a "sophisticated" trader make use of those small transaction fees?