I am not good at explaining things like this but:

Assuming you have now registered for GST, the tax your supplier has charged you is of no importance as it will be claimed back by you. You need to charge your customers gst on top of your price,  just as your supplier has done to you. 

Your price for your item is $250 so you sell it for $275. You claim back the amount of GST you paid your supplier on your quarterly form - and the government gets the extra which your customer has paid as they can't claim back any as the end user.

I think I know what exactly you are asking but I can't seem to find the words to explain it. It is easier to talk to your accountant, or look it up on the ATO website.