We're faring alright here at Bitcointalk without people having to verify their escrow each time they win an auction, mainly because the escrows here can be trusted. So it's not like having a trustworthy transaction with one layer escrow is not practical.[..]Having a service that moderates escrows won't be very helpful because these Level 2 verifiers may themselves be untrustworthy which pauses you to bring Level 3 verifiers to check them, and so on.
I agree a one-layer system may be enough for most transactions with a small-to-medium amount of money involved. However, the problem here can be clearly selective scamming - the untrustworthy escrower would wait for people getting confidence, and in a bigger transaction he cooperates with a scammer (or he stages everything himself using different identities, which isn't that hard in decentralized networks).
Thus I believe that if the "Level 2" authority has some stake in the system - like it seems to be the case with OB1 with respect to OpenBazaar, because if OB1 was known to scam OpenBazaar customers or to be too lax when selecting moderators, then their sources of income might dry up - then it can provide some additional value to the escrow system.
Regarding the Bitcointalk trust system, take into account Bitcointalk has still a centralized authority which acts as a "moderator of last resort", even if the trust system itself isn't moderated directly, the staff can ban those who abuse it too hard. In a P2P app like OpenBazaar it's much easier for scammers to infiltrate such a system (this is at the end why we need Bitcoin and a "money transfer web of trust" - imagine Ripple without a blockchain - isn't enough).
I agree however that the additional information which are possible in Bitcointalk's system (like a link to a "reference" etc. ) would also be very useful for OpenBazaar and similar sites.