...so now when they get back home, and the parent tries to pull them off the internet (or tell the kids to do/not to do anything), the kids are now eqiupped with the skills to kick the parent's ass. Great thinking!

Min-maxing. Get the most out of every situation with the least put in, whether that be biggest return on the smallest monetary investment, most work done in the least amount of time, etc. Make every iota of effort, energy, money, anything you put into the world pay you back as big as you can.

...the ACCC can allow them to only accept PayPal, but PayPal is not allowed to collect any fees on the transactions. If eBay is doing this for "consumer safety", I'm sure they won't mind freely offering their payment service.