Because many players have quit, the economy is in decline. This means there's a large amount of items such as sharks, rocktails bars, ore, etc. Because there are not enough buyers for these products, the cost of the items are falling. This has caused many armour pieces to crash however it will eventually level out.
If you are considering investing, I recommend that you take advantage of Runescape updates. This is the easiest way to start. This is because anti-fire potions are predicted to grow with the QBD and the new KBD drops. Therefore, you should buy lots of these.