The idea of auctioning users is a dangerous game. It reeks of an evil empire manipulation. Social networks are built on trust. The user needs to feel that their community is safe and regulated by ethics similar to their own. A few public debacles involving said user auctions could seriously damage a sites reputation and adversely effect its presence in general. It would be much safer to build in the transactional side of the operation. This wouldn't have to be a ground up operation either, partnerships that would allow a site to reface existing transactional portals would be ideal. For example take a social network of bibliophiles. This example site has a strong sticky content with syndicated book critics and authors along with established "user2user" interactivity. The site then works out a deal with amazon that allows the biblio-site access to the amazon backend for placement and fulfillment of orders for a piece of orders driven from their site. To the book loving user the process seems seamless. This way the site reduces exposure to risk due to bad public sentiment of their intentions and operating procedures.