The person can point to any blockchain transaction that matches the amounts. IRC logs? What do you want him to do upload it to pastebin? Those would be even easier to fake. The OP is almost for sure in on the scam.

Oh guys I just found a way to buy bitcoins at less than half price. I didn't buy more because I ran out of money. Hello! Sell the cheap coins you just bought and repeat. Oh no, I mean I didn't buy more coins because I realized it was wrong. So I posted this message on bitcointalk so other people could find out about it... yeah.