"The buyer" isn't necessarily constant. Suppose Alice values a widget at $100 and Bob values it at $200, and also that Alice spends much more time on Craigslist than Bob. If you sell a widget for $75 then Alice will probably buy it, which results in less consumer surplus than if you sold it for $150.

Buyers have different motives. As do sellers. Mismatches are common, and when people make a trip to someone's residence to look at product in question (as thru CL), the deal, typically goes through, leaving mismatched feelings about the deal.