So, I opened the dispute, and after a few weeks, PayPal emailed me and told me that my dispute was about to expire.
I escalated it to a claim, and then waited for the deadline.
The seller remained mute through this whole process, so Paypal found in my favour and refunded me the $75 I asked for.
Then, weeks later, the seller sends me this:
"would apreciate for you to send back as offered for a FULL refund including post the yz125 barrell piston & head as you were not happy with them to 41 Allison st Bowen hills 4006 thank you"
I've got my $75 back, and I'm walking away from this moron. I actually feel pity for them, because they clearly lack mental capacity. Maybe if the original ad had said "Half-wits trying to make it on our own in the big wide world", or "I struggle to tie my own shoelaces, so the bore diameter I've posted cannot be trusted", I may have have been prepared to pay too much to help them out.
Al's Bike and bits / Patricia Close, FWIW.