Stay clear from electronic items that require direct power plug connections such as christmas lights, electrical fans, etc as most of them don't come with converters. Other items with transformers such as tablets and maybe some mobile phones that'll work in Australia (Samsung, LG) are OK and sellers form Hong Kong usually include a power adapter, but these are pretty cheap from any local shop.
The most important things to keep in mind when buying and dealing with any seller on eBay is:
1. Always pay via PayPal. PayPal can reverse transfers if you do not receive the product and there is a history logged as to who you've paid, etc.
2. Never liaise with the seller externally. Do it through eBay via Messaging so everything is logged for your safety.
3. Do NOT leave feedback to the seller until after you've received the product, tested it and are satisfied that it works. You can e-mail the seller any time within their time-frame after the product has been received. A seller may give you 14 days return or exchange, this applies after you've received the item and I'd highly recommend making sure a replacement item comes first or your money back in the event of faulty items before leaving feedback.
4. Loyal shoppers aren't always going to save. You may have a list of favourite sellers on your eBay. Unfortunately, they may not always give you the best prices on eBay and may charge more for shipping, so it pays to shop around.