[Crazy Dazz]

If you're not good with it yourself, you'll need a bookkeeper. It's not rocket science, and there are plenty of course available. Probably find that various government "help departments" run free or cheap courses.
Pretty sure the ATO makes available its own basic bookkeeping system and offers "training". Do dome Googling.
But that said, it's something you either get the hang of, or you don't.
It's also important that you don't over think it. No Point paying a bookkeeper $1,000, to save you $950 in tax.

Testing and Tagging is definitely big business, but you need to have a very professional presentation, and you'll need to work really hard to get your clients as you will have a lot of competitors.

I don't really know the "markets" or car-boot scene. Most around here seem to have died in the ****, but surely with this emerging "recycle" culture you could make a few bucks selling recycled gear? Maybe get friendly with your local tip shop and/or scrounge verge collections. I wouldn't bother trying to make a "business" out of it though. If you have to charge GST and pay tax, its not going to be worth it.

The only market you should consider for "re-plugging" would be those who have bought (or brought) new (or near new) appliances from Asia, and are annoyed by having to use adaptors.
Not being racist, but I have a few friends who are really fussy about their appliances, and will buy uber-expensive ones from their home country, rather than shop at Harvey's.