Opportunity cost is basically the difference in gains between two options.
Example: you get an item that sells for 5k. You decide to sell it for 3k. By selling it for 2k less than what you could have, the opportunity cost is 2k, which you lose.
As for "high prices never sell," all I have to say is this:
Do note that I've spent about 70k on BoEs for my characters, which is the majority of the spent category.