That said, some major challenges (I've done a bit of thinking about this) - First: Sybil attacks on the reputation system. The author alluded to this but the solution given was vaguely specified and somewhat reminiscent of social network driven overlays like Tonika that a) are inherently problematic for anonymity, and b) hard to grow due to aforementioned invite problem. Another problem is maintaining anonymity while making things efficient - the obvious design would be DHT based but this leads to a whole host of weaknesses (see a paper called 'Hashing it Out In Public' for details), and leads to fragility as DHTs are weakened by high rates of churn, which are quite common in real world p2p networks. Have quite a bit more to say on this topic but I'm on mobile at the moment, may write up an essay at some point.
I would encourage the OP to read up on some of Peter Todd's ideas around fidelity bonds for some ideas on the reputation aspect, also have a look at a (centralized) marketplace called NashX for dispute resolution.
EDIT: also check out the bitcoin-otc web of trust.