The OpenBazaar platform does not have fees and you will not be required to pay any fees but for the escrow service there will be a fee and most people will likely opt for the escrow service.
The developers are planning on offering an escrow service.
So there will be fees, just not as much as eBay and PayPal. And there will be competition for providing that service which will drive the price down.
I do not see any wrong in seeking for escrow. It protects buyers. As you mentioned when more people offers escrow service then we can find a cheaper escrow. Many people hate here escrow. I do not understand why. Bitcoin will grow even with escrow. No need to hate it.
Decentralized eBay, centralized escrow?
What's the point? Am I missing something, but you may as well use an exchange if you will have centralized escrows.
Its a subtle issue but one that is more complicated than you may think. A trustless escrow has its own issues with incentives, for example
1) requiring a buyer to double down and hold coins in escrow until offer goes through and seller accepts/buyer recieves means seller doesn't have to send item, can scam users (especially if he can create new addresses/names/identities to sell on a decentralized marketplace).
2) requiring seller to double down has problems with bad buyers claiming they didnt recieve when they did
3) requiring seller and buyer to double down is ok, but merchants will hate having to hold coins in hostage for process to complete... and what happens with shipping parcel is lost for real? This is where an escrow agent can help to determine best course of action, although if the seller can prove to the buyer that it was lost then the buyer can request a refund and seller can agree and all coins go back to original sources
4) an escrow agent can handle the situation but ofcourse you must trust escrow agent doesnt run off, although he has incentive to not do so to build up his reputation.
A reputation system can arise from the seller/buyer handshake's which might make #3 the best choice over #4, although there is no wrong answer. With #4 you dont have people holding coins in escrow so merchant adoption will be better, but buyer adoption will be lacking cause of complications with dealing with an escrow agent that buyer probably pays for. #3 is free to use, no escrow fees so its the cheapest solution and is trustless so I choose #3 and what I am going to implement in Syscoin's next release on ethereum.