Three examples:
-Reins of Poseidus. I don't understand this one at ALL. You get a mount that can only be summoned in water. The only place you would need this one is in Vashj'ir, and you already get a free one. Why pay as much as 150k for something you'll never use? I can understand TLPD going for that price if it was BoE, but you can at least use the TLPD everywhere to fly, and RoP is only useful for occasionally getting from Stables to BS.
-Rare non-level cap items. There is someone on my server that is trying to sell a rare lvl 58 belt for 350k gold. I've also seen lowbie stuff selling for 50k gold, 70k gold, or more. Why buy something you'll outgear with one Outland quest or a higher level dungeon?
-Non-companion pets. This one I find seriously stupid. I see that damn parrot on the AH for 25k almost every week, and it's just a recolor of the other parrots. Maybe if the pet looked badass or if it did something, it might be worth a few hundred gold, but seeing pets being put up for 10k-30k gold really confuses me.