Spreads on liquid stocks are a penny now. Most limit orders are price improved by a few tenths of a cent -- a better price than you asked for. These are both courtesy of algorithmic trading. If a HFT trader does arb between two diff exchanges he's not hurting you. You're conflating it with front running your orders. That is not really a problem for a retail trader who isn't moving large block trades.
some market players are allowed to trade at sub-pennies, others are not. when one party can trade inside while leaning on the guy taking the risk to make the market, something is very very wrong with the "system". over time, the guys taking the risk are gone, and the whole market is weakened.
getting a price better than your limit isn't better than your asking price. you state i will accept nothing less, which is typically priced lower than or close to the market price. so your asking price is really the market price but with a safety net. it's not really a courtesy at all that limit sales go for higher than the limit.
and oh goodie, i made an extra $50 on my trade because of some algorithm. thank god for that.
Actually, the liquidity providers who are filling orders passed from your broker will give you a better price than the market NBBO (or your limit) by a couple tenths of a cent. You can google it to read more. It's generally called "price improvement". The main reason they do this is to capture order flow so they can use that knowledge in their own trading.
