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
OpenBazaar is not a trading platform to exchange fiat (although you could with fairly high risk). It is a decentralized platform to sell good and services being paid off with bitcoin with 3 optional kind of decentralized escrow system.
It's the same, you bought something stolen, or some one were cheated to pay bitcoin to you and never get any delivery, while the scammer get goods delivered from you. Man-in-the-middle attack will be very popular on this kind of platform when there is no central authority make sure the legitimacy of each transaction