The Rune to Iron Scam is a scam where the scammer is claiming to sell a rune item, usually a longsword or scimitar, for a drastically lowered price such as 10,000 coins. When a victim trades, the scammer offers a noted iron longsword/scimitar, which in noted form is hard to tell the difference from a rune longsword/scimitar. The victim accepts the trade, is scammed and can report the scammer under Rule 2.
An older form of this scam is also where the scammer puts up the noted rune item and switches it with it's noted iron counterpart. However, due to a new system where quick changes in trades are pointed out by a large red exclamation point, this is much much harder to pull off, if not impossible.