Hey BarnOwlBy! Great to see someone else starting down this path.
My number one caution to you as someone who hasn't used Pepakura before is: you may want to find some random pep piece to build that isn't a helmet on your first shot! Seriously, helmets have incredible detail and are arguably the easiest peps to identify mistakes in. I might find a gun or some other pep piece to build before the helmet, just because you will learn a LOT in the process of building your first piece. You don't have to go through the process of actually hardening it as well, but I would at least get a few hours of cutting/scoring/gluing in before setting the helmet in your sights.
My thread on the 405th of my MJOLNIR Mark VI (Master Chief) build would be a decent place to start for advice on pepakura basics!
As for the comment about eBay, I think the guy was saying that Pepakura files regularly find their way onto eBay. I can't say I have looked for hand made helmets on there, but I know I wouldn't sell anything I made on eBay -- not worth it!
Best of luck with your build!
Thanks for your advice! I'll go and have a look for some more simple pep that I can have a go at. Best of luck with the rest of your build!