Really, the only change I see here is the cost of posting a item.
Before the 1.5 patch, you posted an item and paid a big fee to post that item. The 'tax' you speak of was deducted from the sale price and deposit combined. So before 1.5, (numbers made up) you sell item a for 10,000 credits and pay a listing fee of 1000 credits. When the item sold you received 11,000 credits minus the GTN's commission. This figure always exceeded the 10,000 credits you posted the item for so for example you would receive 10,500 credits.
With 1.5, you post an item for 10,000 credits but only pay a listing fee of 100 credits. When that item sells you receive that 10,500 credits but minus the commission. Therefore in this case you would only receive 9,600 credits. Therefore the only change is in what you pay to post the item and you are only noticing the 'tax' because of that reduced fee.
R The commission has always been there. The only difference now is that you are putting down substantially less to post the item so you are noticing it whereas before you did not.