Unfortunately I see a second problem with these decentralized marketplaces, and that is trust. <...>
that's a good point which i wasn't thinking about, whenever i think about decentralized marketplaces i see them from the perspective of the seller/shop not the buyer/customer. @stompix pointed this out to me
in another topic.
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.
PS: Just got an idea I may develop further: One of the problems of decentralized marketplaces seems to be the communication with the postal service, because it's difficult to prove a vendor didn't send the correct item. What about a crypto-friendly postal service which offers an option to verify the content of a shipment? Problem may be that such a service could be costly, but what I have in mind is that the vendor should pay an additional fee which he gets back if the buyer doesn't request dispute resolution ... Is for now a very immature idea, maybe it already exists.
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.