When i compare centralized with decentralized one question comes in my mind that is really decentralized system is viable to do transaction without knowing any central authority.
This information is incorrect, there is nothing the government can not identify. Unless the parties use currencies based on privacy "Monero - Zcash".
decentralized system whom we follow and if something fraud happen and whom we target ?
Many of these sites have a secure escrow service, so using it can reduce your risk of being scammed by a large amount, also this scam may occur in the central platforms.