The slogan comes from the CluetrainManifesto. It means that people value discussion in human voice and that this is the original attraction of the market.
The significance of the slogan is greater than this. MarketsAreConversations both suggests and transcends the well understood capitalist idea that markets are engines for processing information - they help people exchange ideas about price hence distribute information about the availability and demand for stuff - and reconceives them as engines of SocialCapital, networks of connections. Because conversations also construct more general social relationships.
This broader understanding, that networks, in general, create value, fits in with the transition from Capitalism to Informationalism (eg. see NetoCracy, PostCapitalism, FeudalismCapitalismInformationalism, also NetworksCreateValue)
- SchoolOfEverything on paid markets for teaching : http://schoolofeverything.com/blog/howilearnedstopworryingandlovemarket
The other point of "markets are conversations" is that your trip within the market has FractalLoading. Your exchange is wrapped in extra layers of communication. And, of course, the same goes for voice, corporate blogs, GonzoMarketing etc. All fractally load secondary information on top of your bare commercial transaction.
I wrote this up more formally on my blog : MarketsAndFractalLoading
Scribe wants to Haggle : http://describe.blogspot.com/2008/02/wheres-my-e-haggling-e-market.html
SpringWise on Generation R : revenge consumers : http://www.springwise.com/newbusinessideas/34007.htm
See also : OnMarkets