In your end you need to record sales of $110 with $10 payable in activity statement. Ebayon the other hand will record the $22 as their marketing expense. I don't think they will be eligible to get $2 GST credit though as they don't get tax invoice from anyone.

Anyway there is a way to know if buyer uses discount coupon. On your order list do a pretend cancellation, then on next page it will be the confirmation page detailing buyer payment including voucher used.