I got a few where people were mad that I simply re-listed their item at a higher price than they sold it for. I'm sure they felt duped for undervaluing their product in the first place. As binarysolo pointed out in this thread, some sellers feel they need to urgently sell their items, so they under-price. I'd could afford to wait for someone to pay my asking price.
Instead of being angry that they didn't maximize their profits, could it be that the sellers were _offended_ that someone would maximize profits with their possessions? I can imagine a couple reasons:
1) The Toy Story 3 plot: people want to find a home for their possessions with people of similar interests. They become angry that you are reducing the consumer surplus that would have gone to the ultimate buyer.
2) They realize that arbitrage on a first-come-first-served marketplace can reduce the value of the marketplace as a whole, since buyers are less likely to find bargains. They recognize your behavior as antisocial and become angry.
The reverse can be true for both of those points. Raising the price to the market-clearing level increases the probability that it gets sold to someone who places a greater value on it.
"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.
