Sounds like you might have encountered either the dreaded rear-door leak problem, the blocked plenum drain channels, or the debris in the pollen filter problem.
Have you checked for dampness or water in the rear foot-wells?
The rear door leak is curable; Skoda now produce a kit to seal the rear door-carriers to the door (the gasket between the carrier and door seems prone to perishing).
The plenum (aka scuttle panel) drain channels and pollen filter are just regular maintenance issues. Check them and clean them.
I'd ask that the guy acknowledges the problem with the rear doors (and the rear washer pipe in the other Octavia thread) and agrees to fix it if it crops up.
There is no real "in-advance" treatment for the rear-door leak - if you go messing with it you're every bit as likely to cause a problem as to prevent it. Keep an eye out for water lodging on the bottom of the rear door-seal.
Hope you get on OK, they're great cars - I'm a big fan.