Hi Adrian,
You can pretty much trust any seller that has an IMCA number and is a current member. There are several people that do sell them, but not too many in Australia compared to places like America, unfortunately. Some people you can trust are the well known meteorite men.. Geoff Notkin has a website with meteorites for sale..
http://aerolite.org/ There is also Meteoritefinder (flattoprocks) on ebay who has some very nice specimens
http://stores.ebay.com.au/meteoritefinder. There are others also, but check to see if the seller has an IMCA # that will help you work out if the seller can be trusted.