Difficult one this.
My take is that if you bought something by post and it was defective, or not as described, then you'd have legitimate grounds for a moan.
However, this time the buyer has actually tried the rifle out before buying it - and so has had the opportunity to satisfy himself re it's condition.
The problem described is one that no one could have known about without chrono'ing the rifle anyway.
That's the chance you take when you buy anything secondhand - that's why secondhand goods are cheaper. Really in this case the buyer is asking the seller to guarantee the goods - but if you want guarantees you have to buy new and pay the full price.
I don't know the full ins and outs of it but from the above I'd say it's really the buyers problem - he obviously thought it was OK when he tried it out.
But I stand to be corrected!