He was just planning to look up, if it's a legit seller or if the seller lied to him.
The history of the coins being sold is not correlated with the legitimacy of the seller. As I said, I'm certain I've bought and sold coins which would be classed as "tainted", but I have never scammed anyone.
Because if the seller lied, it's a big red flag.
Why would the seller even know the history of the coins being sold? I don't know the history of any of my coins, nor do I care.
That's no issue because from what I've read, the buyer was going to request which UTXO's are for sale (or at least from which address it's coming from), the seller sends his address, where the BTC is coming from and then, he (the buyer) could check on his own if he wants to make a purchase or not.
If I was doing a trade and the other party said "Send me your address/UTXO before we trade so I can spy on your coins", I'd simply end the chat and find another person to trade with.
If he's getting other BTC than previously agreed to, then he could refuse to pay.
Then he will be at fault for failing to complete the trade. He'll either lose his security deposit or be banned from the platform, depending on the P2P platform being used.