Well there are some advantages with this which could help prevent scamming. Lets say you wanna trade a speed +1 ring for a dex + 1 ring. They are both valued at approximately 100 plat. To ensuure that you don't get scammed the person giving the item first (let's pretend it's the speed +1 ring) is given 100 plat then when it's time to recieve the dex + 1 ring they give the 100 plat. Even if someone backs out from a trade after they recieved their item you get to keep the insurance which is an average/approximate item value (100 plat in this case) so you don't lose out
I actually like it this way.