Deja vu is strong with this thread.
Personally, I sell an item at what its value is to me, sometimes that's less than what I paid, sometimes not. If the buyer agrees with that value, I sell it. If not, I either negotiate to an agreement, or I don't sell. Seems simple enough. It takes two to make a baby, as the saying goes. Occasionally, if its a friend, a longstanding forum member, or sometimes when I'm just feeling jolly, I'll give an additional discount.
I have two basic principles. One, I'm honest in my sales thread descriptions about the items, their condition, etc, and in reply to people who reach out with questions. Essentially, I do my best to help people make an informed decision. And two, I don't take advantage of newcomers. If they still want to buy once they fully understand what they are getting themselves into with their potential purchase, then that's their choice.