In this business your inventory is stocks, bonds, futures or options. Like any other business you define what an acceptable loss is on an item and what is an acceptable profit for the risk undertaken. Like any other business if the item of inventory doesn't do what you expected it to do, you put it on sale and liquidate it to raise capital to purchase inventory that will do what you want it to do. Your acceptable loss is your stop. Your money management system tells you how much that is. Your mark up is dependent upon your trading system and trading style.
It doesn't make any difference if you are a day trader or an investor. Like any business, some turn their inventory 10 times a day, some 20 times a year and some only twice a year. Your trading style and inventory volatility will tell you what your turnover rate will be. Trading is a business and if you treat it as anything else you will be a loser.
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