IMO, decentralized trading platform has been promoted for some years but never gained enough traction. This is largely due to the fact that no one want to send the money/goods first, and there are many complex scam/theft schemes in payments, which makes the dispute impossible without the involvement of authorities, but when you have authorities that can rely on, you don't really need decentralized trading anymore
This is not a valid point. first decentralized trading platform have never been fully implemented before. You should read more about OpenBazaar, they received 1M$ investment and launching this fall. Joystream is another different beast where you point really don't stand.
A simple example: Someone bought 1 bitcoin from you on OpenBazaar, paid $230 to your bank account, you release the coin to him. The next day, you found out that your bank account is frozen and police called you for an investigation, because the money is hacked from another bank account
How can you mitigate such kind of risk? You must make sure the money is sent from the users own account by asking him to upload his account screenshot together with his ID card. But normally users will refuse to show their ID card to a stranger on a decentralized platform, they would only show it to a well known registered finance institution which have strict rules for customer privacy protection