HandBrake:
I did a clean install of Windows before selling it though wouldn't have thought it would cause any issues (I've always done this for previous laptops/desktops I've sold and never had any issues reported).
Will ask for a copy of the event logs and try to assist to avoid a return. Will say it does suck for them... I'd personally hate that if I was on the receiving end. The guy has 100% feedback and lots of trades so didn't feel he would be the type to try and pull a fast one.
Will see how it goes. Cheers.
Did you stress test it again after installing windows? Did you install the required drivers for the laptop?
Sounds like it might just be a software issue. - ask him to send you a video so you can get the BSOD info.