I just submitted a Best Offer on sumfin and it specifically told me that my offer was for the item only.

So by Civil Law standards i'e it tells you what you CAN do, that means that any reference to include postage costs etc as a condition isn't gonna fly.

An Australian seller's terms and conditions don't trump eBay's policies or ACL.

The wording on timeframe is less specific though - it does say "you "should" allow up to 48 hours for response.... the term "should" is subjective and is not obligatory, so I can't see a problem with including that and eBay obviously agrees as they allow you to withdraw an offer, as does the law - ie - an offer can be withdrawn at any time before acceptance.