in any case i think this issue could be partially solved by a rating system. for example an old shop that has been around for a long time could have reviews and points given to them by their customers. it could even be stored in a blockchain (maybe a side-chain even). although it is not a fix but it can lessen the severity of issues.
The problem in this case is obviously the same as with Amazon: fake reviews. OpenBazaar, as far as I know, already has a customer review system. But it's hard to imagine it cannot be abused, if the seller can simply sell goods to himself.
Maybe it could help if the customers are also reviewed by the sellers - while this can also be abused, it would be easier to spot "circlejerk" networks of identities constantly reviewing each other but never receiving a review from outside of that circle. But even in this case, selective scamming could be a problem.
What about a crypto-friendly postal service which offers an option to verify the content of a shipment? P
i think that requires some sort of record of such trades which also goes against the privacy that people may seek while using a decentralized marketplace instead of a centralized one. this could also bring regulations and taxation and with that limitations.
The kind of postal service I have in mind would be a regular company, which would be responsible to protect the data of the customers (buyers and sellers) just as all these services are. Basically, the additional service would consist in something like a photo being taken of the content of the package by the postal service, which would be shared only with buyer, seller and escrower (obviously, both must agree to that). The postal service itself wouldn't be involved in the payment process so it would not be obliged to register as a "financial institution".
Obviously this limits the usage to legal and "unproblematic" goods, and it can't detect all sorts of scams (e.g. sellers sending defectuous electronic devices) but it already would offer more security than a simple tracking code. And it could be combined with e.g. the seller providing the option to return the shipment in the case the buyer doesn't like it (well, in theory in this case the buyer can scam the seller, but then again, this service of taking a picture of the content of the package would make some scam attempts (e.g. returning an empty package, or an inferior good than the one originally sent) impossible or very hard without being detected.