Fast payment, friendly emails. Would recommend to any seller. Observe that, from an economic standpoint, the feedback mechanism exhibits externalities. Those people, moreover, usually have no way to determine whether my feedback is accurate. I am providing those people with free information.
Simple altruism is part of it, and curiosity about the workings of the new medium.
The two parties have a pecuniary interest in collaborating to provide a misleadingly positive view of one another. After all, as Kollock points out, positive feedback can increase the prices one gets for one's goods. And my experience is certainly that this kind of collaboration is routine, and even overt. If a buyer feels abused by a seller, negative feedback provides a mechanism for warning other potential buyers of a potential problem. But the buyer receives no benefit for issuing this helpful warning, and at least two kinds of incentive not to.