The selling price is no longer the price that will be paid. You're now charged by the average of the buy and sell offer. This means that if I buy an abyssal whip for 2m and somebody sells it for 1m, it will cost me 1.5m and the seller will also get 1.5m ((2 + 1) / 2 = 1.5). This system ensures more fairness towards sellers and will no longer result in hesitations for sales.
I don't see how this helps anyone other than people, who forget to press k or m while typing the selling price and post the offer without realizing it, those people would at least be getting half of the money they could've gotten.
Sellers who are desperate for money, will try out different values anyway, so it makes no difference for them.
Buyers who are desperate for items and are willing to overpay, will however get screwed over every time, so I can only see this having a negative effect as people will more hesitant to buy items in GE, which is bad not only for them but also for people who are collecting those items in hopes of selling them.
I can see this further decreasing the demand for items in the market, which was already too low.
Also if I'm selling a ton of a certain item, let's say coals for 1.9k ea and people buy them 2k ea (which they probably would if they did some math to estimate how much the coals that they need would cost them, as they would've likely rounded the price to 2k in those calculations), then it doesn't really make sense for the price to not be 1.9k when the best buy offer and the best sell offer are matched, because if the price balances at 1950 gp then people trying to sell them for 1950 gp would never sell a single one, because my 1.9k is better than their 1950 gp, so GE would be misleading them about the price that they should use on their offers.
If on the other hand the price was always my price, 1.9k and GE price and the GE market watch thing on the homepage would show that, then someone wanting to sell their coals could sell them for 1.8k or 1,899 gp as then they would know that they could expect to actually sell their coals, without being confused about whether there's no demand or whether all the transactions going on are between people selling for 1.9k and people buying for 2k.