Perhaps they'd detected that the character selling goods was a mule, perhaps they see the AH as a NPC market with fluctuating prices, maybe anything that isn't player-killing is an inferior activity in their minds. It's interesting to think about how we see the AH ourselves, and the opinions we have on those who play the system for maximum profit.
When saving for my first mount, I farmed some high-price herbs and ores, and had a mule sitting in the AH for about a week in real-time buying low and selling high. That character still has a bank full of stuff I didn't manage to sell at the prices they "should" be sold for, but I got my mount money quickly and easily. Is that an ethical way to play? I can do the same on eBay, so why not in-game? Perhaps knowing when to stop makes a difference.