There is an audio interview with one of these "cashiers" who explains it all (you have to fast forward a bit): http://smallworldpodcast.com/mp3/smallworld051906.mp3

It's fascinating. Basically, the cashiers are working on the honor system. They get to keep a percentage (this guy gets 40%), and they send the rest to the "boss" through e-gold. The percentage you get increases as you earn their trust. I guess that this keeps most people honest, as they want to get a higher percentage the next time.