It only takes a matter of minutes to register for GST when the time comes. 

What do you mean by "more to it regarding tax refunds"?

There are a lot of charges in business that have no GST but are tax deductable...just off the top of my head, bank fees are a deduction but have no GST on them.

As for your hypothetical question....if you are registered for GST and are charged GST on something you have bought to onsell, you will claim your GST back from the Government (via your BAS) and will then add it to the item when you sell it.

The GST is paid only once, by the end user/buyer.  If you happen to sell to someone who is also GST registered and he then wants to onsell it, he will go through the same process of claiming the GST paid to you, then adding it to his customer's invoice.

I can not speak with any authority about import duties but I believe they would just be treated as a cost of doing business...tax deductable but not claimable as GST, which is a different charge and already claimed on your BAS.

Neither of the businesses that I do the BAS for are involved in importing.