As a general rule, when people are selling anything, it is up to them to research the market and decide how much to ask for whatever they are selling.
I can understand your dilemma, because you feel that you have got a bargain, but would you feel the same way if you had been paying the asking price for say a car, or indeed a house?
Years ago, a very seasoned dealer whom I knew, always used to advise that in arriving at a price, "one should always leave a profit in it for the next man", so perhaps the seller knew what they were doing!