This is what we're working on now.

But trust is a harder problem.  Yelp doesn't improve the trust of brick and mortar stores, the bricks and the mortar do that.  The problem with web of trust is that you must understand what you're trusting.  With Public Key Infrastructure (pki) the web of trust is only Identity.  That is you can 'trust' that the key used to sign one document is indeed the key associated with a particular public key or the same key used to sign a different document, and to the degree that others have also signed the public key, they assert that the person is who he said he is.  This is not the same as saying that the person wouldn't steal from you.

As ebay has proven online reputation systems are difficult to build.  It's always easy to build up a lot of fake reputation, simply to do one scam after which you don't care if you get a single bad review.  We are working on designing a system that is weighted so that it takes many successful transactions to build reputation, but each bad transaction makes a huge negative impact.  We are optimistic that we have a good solution, but we still have a fair amount of work to do before it's ready for prime time.

Meanwhile use bitcoinlocator to trade in bitcoins face to face where trust and identity are much easier to deal with.