Just when you thought there couldn't be any more threads re: ebay and GST.... 
I promise it's not about OS purchases, although realising that eBay pays GST on local purchases via discount vouchers but not on OS purchases is what made these thoughts invade my head - also, I registered for GST last month, so I have a ton of new things I have to consider that I never did before. 
Ok, so, enough pre-amble... As a GST registered seller, I now have to provide my buyers with a tax invoice. That's fine and simple, but, what about when they use a discount voucher, yet I receive the full amount of my asking price?
In those cases, the buyer would pay up to 20% less, and unlike with overseas purchases, where the GST is calculated separately and discount vouchers don't get applied to the GST component, the shortfall that eBay pays to make up the full amount means they are also paying 20% of the GST, because it's included in my prices. Is it even legal for me to provide a tax invoice for the full value, when the buyer paid less, despite me actually receiving more? 
From the collection perspective, that's not an issue, but from an invoicing perspective surely it has the potential to be incorrect and therefore cause issues, even if but maybe only for the buyer if they are also GST registered (which I know some of my regulars are)? For the sake of ease, if an order is $110 in total, and I've collected $10 GST, then technically eBay has paid $2 GST to me (unless the buyer can tally it differently, and say they paid $88 in total, which includes $10 GST? But then won't that imply I'm over-charging GST? Ugh... ).
I have a tendency to overthink these things, and maybe the answer is simple, or it's not even a problem, but it's one of those small, sudden thoughts that I know it will bug me until I know what the kocher way to approach the situation is, if there is one, as buyer and seller, so if anyone has any opinions or insight on this scenario, I'll welcome it.
ETA: I fogot to mention that there is no way for me to know if a buyer has used a discount code or voucher anyway. Which makes the whole situation worse! 
