How come when you have someting to sell it's always worth less than when someone else has it?

I had a person stop in my lawn sale and was interested in a Depression gl bowl. I wanted $10 for it, but they said it wasn't worth over $2, so we compromised at $5. I know this person runs a second-hand/collectibles store, so the following week I visited it and saw the bowl I had sold marked $50! That was quite a mark-up. If I hadn't sold it for $10, I doubt it will sell for $50! I just wondered at this constant put-down of prices. People wanting to buy houses do it too. If you have a house for $150,000, they will ask, "Would you take $50,000?" When I go to sales and see something that I consider over-priced, I just walk away and don't haggle over it. If it's a price I am comfortable with, I just pay it and don't ask for a further reduction. Some people will argue over ten cents!