I am aware of a few people on Ebay who make an alliance and post together to get the business discounts from Aus post. 

Not sure how that would work, as I assume you do need an ABN etc, which is fine, but will probably make things weird at tax time if you run the two business seperately, especially if youre audited.

Good idea, just do it with someone you trust.

I think theres also a Auspost 150 special...? not sure i think Digital Ghost mentioned it previously. Not sure if you need to be a business.

There are ways to make EVERYTHING in life cheaper without changing format (eg becoming a business), or changing companies (e.g using a courier). You just need to find it. Trust me, it's out there. 😉