It is a teaching tool. It helps get the message across to make the model simple.
Because in not doing so it means they place no value in their own product/trade/work. If you believe what you do is worthwhile, charge for it.
Your worth to yourself has nothing to do with profit and loss. It is the worth to the seller and the cost it took you to get there. If you are unable to make a true assumption of cost then you are guilty of being a "looter" (in trying to get more then your fair share or your worth). Not to mention, you won't sell shit and will price yourself out of the market...not a good way to pursue profit...meaning you wouldn't be. You have to set real pricing expectations.
You are only losing that money if there is someone else willing to pay you more at the same time (under the same conditions). If the opportunity is not present it isn't an opportunity cost (= the cost of a forgone opportunity).
You either don't understand profit/loss or the other poster was right and you're just really good with the passive aggressive B.S.
Making up symbolic transactions and ignoring other transactions is not good business and is not pursuing profit.
Which is a very Randian ideal. As I mentioned in the previous post. The biggest part of her philosophy (and the most applicable to the real world) is rational selfishness. The pursuit of profit is a very black and white simple model to work with for self value and worth. It is not the only interest that one can pursue. If you want to delve further, try The Virtues of Selfishness next. It's MUCH shorter then Atlas and is good for a quick recreational read since it's a collection of short essays.