I just traded a Noxious Longbow worth 135m for a bunch of elder rune 2h swords +4 that are apparently worth around 8m each. So why is the game telling me I made a good trade when in fact I basically just gave away my Noxious Longbow??
Never, ever trade an item you don't know how much is worth to you.
Grand exchange prices can only update at the rate players are buying/selling them on the grand exchange. And if nobody is trading them on the grand exchange the price has no reference value to adjust to.