There are many reasons. Here is one: knowledge is power, and it can be used as leverage against you.
My friend sold a Gottlieb wedgehead for $3,500. It was not an especially rare title. He liked this game and didn’t want to sell it, but the offer on the table was irresistible. He took the money and set out to find the same game again. He kept this windfall sales figure largely to himself for obvious reasons: he doesn’t want to negotiate with another seller thinking their game is worth $3,500.