During my recent honeymoon in Egypt, I had the opportunity to experience some true social marketplaces. These were actual physical marketplaces, where everything is negoitable.
While we spent most of the time in the more tourist-oriented sections of markets like the Khan El-Khalili in Cairo, the social aspect of these markets was very refreshing. Every purchase involved a discussion of quality and haggling over the price. The beauty of such social transactions is that the final price is set by both the buyer and the seller - everyone is happy. Even more so than this, the social aspects of the experience were fantastic (with the exception of some of the more aggressive vendors, that is). You could actually engage in conversations, not always about the product being sold.
This social experience of commerce is virtually non-existant in North America, replaced by set prices and sales associates who know little about the products being sold. The web presents an opportunity to bring back the social marketplace - but this time on a global scale. Conversations and negoitations can place oceans apart. Friends can be made, and reputations can be built and ruined in the process. We can learn the experiences of other buyers of the same products and services and compare prices of international vendors easily. None of these things are easily achieved today. But as software, systems, and social networks mature, a social marketplace of global proportions is possible.