I was browsing the Trading post and was looking at iron ore when I noticed that the lowest listing was 14 copper and that the highest offer was 15 copper and the second highest offer was 12 copper.
Basically items on the TP looked like this
Buy Orders for iron ore:
15c
12c
11c…
And the listed items for iron ore:
13c
14c
16c…
Now you would think that all/some the iron ore priced at 13c and 14c would be sold to the buyer who placed the order at 15 c, either at the price they were listed at or at 15c an ore. But neither happens, so the order that is at 15c won’t get filled and if people don’t pay attention and just blindly match the lowest selling price then it will circumvent all the 15c buy orders and keep filling up the TP with items priced less than the highest buy order.