1) In most cases (I assume you are not an addicted maniac that sits all the day at the computer) you have to fit your selling scheme to your log in/checking times. Using the method described by Knolls you can estimate how much REAL LIFE costs you in terms of lost in-game margins. Given the relatively flat profit curve shown in the example - it turns not so very expensive

2) If you are a self-producer you have to optimize your profit of your production-selling combo. Your calcultation will be more complex and your optimal price will be significantly higher than in case of pure retail business. But in this case the abovementioned RL constraint also applies.
Just my few cents.