This may not be viable all the time, for some items, you may want to sell to some one whom you do not know. You may not want your friend or college know that you owned such an item.
So what is an ideal situation? A selling platform, which is simple in interface, allows social connections via famous open ID method, provides capability of seller and buyer to choose whom they want to interact with for a particular transaction. If this is incorporated into craigslist or ebay that will be like a tasty icing on the cake!
Yahoo groups is an interesting data mining attraction. Why are people forming groups? What are their common practices while forming groups? I do not have the data from Yahoo or Google groups but my guess is that many of them forming groups to discuss or plan group purchases or to discuss whatever they purchases recently. This could be electronics items, homes or cars. If we surf through many of the high volume discussion forums in the internet, we can see many of them having a board or section for sale. Yes, to handle sales between their members.
Selling socially is everywhere, it is finding its way to the mainstream, but very slowly.