This has been the rule since I the time I joined TE. However, mods weren’t too strict as long as the seller declared everything honestly and acted in good faith.
However the important bit is this:
If you are posting something on TE, it is up to you to uphold the terms of the transaction that both parties agreed to. What happens between you and your friend or what policy he has is irrelevant. Even if your friend does not return the money to you that does not mean you can get away with not refunding the buyer.
And what’s this “trust me bro, 1 month warranty” nonsense. If there is an issue with the PSU or cables, his other parts are at risk. Do you expect the buyer to believe that someone who is acting shady before the deal has even concluded will assume responsibility and pay for replacing the affected parts? You must think people are idiots.