well I think that as long as you do your research eBay is fine. With all of the measures that they have in place I think its a pretty safe bet. But, if you dont want to use eBay than thats up to you. As for some Australian place having parts... I cant help you there as I live in America.
Also... Im sure that if you just wanted to try ebay that you could get a K6-2 mobo for like $10 US. I dont know what that translates to in Australian money, but thats not much over here. Basically, if you did bid on something and win and for whatever reason you never received it, you would only be out about $10-$15.