As a p2p trader most merchants doesnt know the source of the coins they are buying through p2p that's why, they fall victims too stolen coins, its a tough world out thier and everyone is out to make it in life not minding who pays the price, well as for me am moving all my funds away from all US exchanges since i cant categorically know the source of all the coins am buying.
Also my advice too p2p traders is too stop recieving coins from Unknown source straight too ur exchange wallet as all exchange wallets are monitor and not decentralized...
If you're receiving funds from unknown wallet, don't send it straight to an exchange wallet
yea on chain it cant be flag, plus using shuffing services to hide the source or trading in dex