Also where are these reward points from?
Does he really needs to justify to anyone raising questions ? I think not.The seller,the buyer and the involved members who are directly or indirectly part of it deserve an explanation but not random members who would jump into an argument asking for proofs when those proofs wouldn't do them any good.Even if he successes to explain the sources of it,you will still try to dig deeper to know the legitimate root of the source.
I'm not stating the accusation is wrong or the seller is correct but the justifications as requested by you should be asked by the concerned members as they agreed on a trade.Additionally,what would you do with that information anyway ?