IMO it is the buyer's responsibility. Consider a scenario: the seller gets a condition report when they first advertise the plane, but for whatever reasons, it takes 12-18 months to get a buyer interested. What then? The condition report is so out of date it is meaningless as all the info about hours, maintenance, landings and even possible damage is likely incorrect. So when selling a plane, when/how often would you get the condition report updated, bearing in mind that it costs money every time you do it?
On the other hand, the buyer and seller can agree on who pays for the condition report, but again IMO, if a buyer is not prepared to pay for a condition report, I'd wonder how serious they are. Edit: in fact I'd go so far as to say that if a buyer is not prepared to pay for a condition report, I'd regard them as a tyre kicker and treat them accordingly.