The bid-ask spread on used physical goods

On the stock market, prediction markets, and elsewhere, things don't have One True Price (or, therefor, one true valuation). There is the price people with the thing are willing to sell it for, and there is the price people without the thing are willing to pay to get it. These two prices are generally not the same, and the gap between them — the bid–ask spread — is often wide.After decades of considering myself a fairly canny bargain hunter and negotiator, I noticed something: if I bought something that was “an incredible deal”, and then tried to sell it, I generally lost money. What?! Isn't the idea that something is a "good deal" if the asking price is below its value? And if its asking price is below its value, doesn't that mean that you should be able to sell it at its value?No no no!All this time, my mental model of the pricing dynamics at play was completely wrong. Because the bid–ask spread is everywhere! The bid–ask spread generally represents the cost of doing business. In…