This just seems like a way to give advertise that you are selling or buying bitcoin using a blockchain with a reputation system without even checking if trades even happened before reputation was given. In other words, reputation system could be abused.
Also it would be cheaper to just implement a P2P network for this, as none of it requires a blockchain. Blokchain is good for timestamps, to insure what went first, so double spends can't happen. It doesn't really bring anything here.
So it is an interesting idea, but wrong use of it. To be reasonable you need something that requires ordering to be done. Blockchain is good at ordering things in a decentralized way.
It is centralized in the sense that everyone can agree that this is bitcoin's main decentralized p2p exchange. It doesn't rely on any dev's or websites that can be shut down or anything. It allows wallets to encode functionality to auto-trade mined coins if the user wants.
The reputation system I am proposing is a #FB (feedback) message that only is valid if the wallet signing the feedback message was the one involved in the bitcoin transaction. So it is unhackable.
Then it is just like web of trust, which again, has no need to be implemented in the blockchain. It is too expensive and has no added value.
I don't like clicking links from random people on the Internet, so I am just going to say that you might be interested in a project like Bisq (former Bitsquare). They use multisigs and security deposits to make decentralized exchange (fiat<->crypto trades as well).
I read about bisq. It is complicated. It requires traders, contributors and stakeholders, and defines several categories of high-trust bonded contributor roles such as maintainer, operator, and administrator. Also it is an application and depending on which platform you use (especially mobile) it can be censored by app stores potentially. Also it has it's own token, so you have an entire other cryptocurrency involved.
The main benefits of the op_return idea is that everyone that uses bitcoin can just use a block explorer in a "google" fashion to access the built in exchange. Not only would it be a p2p decentralized un-censorable, crowdsourced (no dev's) and private no personal info required exchange, but it would be the revolution that would make bitcoin and the blockchain mainstream. Everyone would understand and be able to use it without the need for any other software and bitcoin will be much more widely used. Also it opens the possibility for automating exchanging of coins and fiat since wallets can rely on the op_return exchange as it will never be deprecated and isn't dependent on any dev's.
Bisq is a serious project so developing it in a decentralized way requires DAO and certain roles. All those roles tho are mostly outside of the app, there is no one that has access to all trades or something like that. The app is decentralized it is just that Bisq has a forum,website,developers and many more that it also wants to decentralize, so they take on the roles in the DAO. The cryptocurrency BSQ is optional and it is there just like a app coin that is there to fund the DAO, other then that, you can ignore it's existence. It also doesn't even exist on mobile, just PC and it's distribution is the same as Bitcoin Core for example. You can compile form source, download from Github, download from website, etc.
It is a cool idea that you have, I just think that you are not there yet. There is really no reason to put this on blockchain. Sure it would be cool, but expensive and unnecessary.