Let’s help out any noobs that come here looking for wisdom. We’ve all made stupid, stupid mistakes at some point.
I’ll lead the way, with my tail between my legs:
I had just learned how to transmute Heart of Fire into Elemental Fire and was buying up mats in the AH. I had recently sold a stack of 5 Arcanite Bars and had a ton of money. I saw several listings that were cheap and selected the top listing because the whole area was riddled with 80s-90s listings. Unfortunately, the top listing was a bid of 79s and a buyout of 90g, not silver. I accepted my buyout, assuming it was 90s. Ouch.