Well in this thread alone three people have bought or saw the same card sold for $300 or less within the last couple of years. It is just my opinion ok that I would say $300 or a bit up is a fair price. To say two people offered you above $400 and you knocked them back because you want $500, well I think that is not fair on the people offering that sort of money, above and beyond last known sales. I don't see how being the buyer or seller has anything to do with determining what a fair price for an item considering you came on to an open forum asking for opinions.
Perhaps you should have rephrased the question to what is the highest I could get for this item?
I pity those who wanted to pay in excess to $400 and you wouldn't budge to accommodate.