Im trying to buy a .com.au domain but they ask me for my company ABN # or TradeMark #. What can I do??

No, that is very illegal. You can hire a company in that area, its called a SubContract, then they can apply for a Domain, thats called doing business. I have a chap in the UK I send $50 a year to, he works for me. He can apply for a .uk Domain, I cannot. That is how things work. As long as you are spending money and paying taxes, all will be ok. There are rules and laws that are meant to either be obeyed, or worked around, that is a Standard of American Bussiness.