The issue with a completely decentralized marketplace is the potential for scamming. I think the idea to avoid that is an escrow system where both partys + a third party have to agree the transaction went through with the alternative being the escrowed money just gets destroyed or is donated during a failed transaction.

I think Open Bazaar is going in the right direction and then you just have to trust the feedback system.

I imagine 9 out of 10 times everything will go great but there are bad actors out there.

Because so many people are working on decentralized marketplaces I've been developing a centralized marketplace this past year. We recently launched at Coinsen.com Being centralized we're able to offer dispute resolution, buyer guarantees, and more integrated features for the whole buyer/seller process.

As a side note I sell on eBay/Amazon myself and remember losing money in my early attemps when I didnt realize how high the fees were.