Failing to find what you're looking for because of a frustrating UI is not positive reinforcement. People stay on the site because it is the largest marketplace and they have no alternatives.

That's weird, I've never had any trouble finding what I was looking for on Craigslist. There definitely are ways that they could improve the search interface, but the current implementation works fine for me, and given their user base it works well enough for a lot of people.

It's actually much more variable reinforcement - of course, the most addictive kind.

Each individual search has a low probability of success, but occasionally you'll succeed.