I guess the edge that centralized marketplaces have are consumer and seller protection.
eBay's Money Back Guarantee is far from perfect, though it ensures that the seller delivers what the post says. Same goes to Amazon. They also have protections for seller in place in case the buyer deviates from good buying practices. So far, I haven't seen decentralized marketplaces offer such protections from both parties, and perhaps that's why people will still choose to buy and sell on these marketplaces despite the high fee.
People tend to always play on the safe side, and employ fail-safes in order to not get burned fully. It boils down on what the consumers and the merchants think, and decentralized marketplaces don't address that directly.
My thoughts exactly, irrespective of how imperfect the money back guarantee may appear it gives some level of assurance that you will either get what you ordered for or have a complete refund in the event that you receive something different, same can not be guaranteed on the decentralize market place, nobody wants to buy in an unregulated platform, this is an important aspect that will always give centralized market place an edge of the latter.