Just found this out the other day after being a victim of it myself. I was going through the auctioneer auction search list as usual and found a brilliant mana oil for 15g (normal price on my server 30g), so I brought it. Now a mana oil starts of with 5 charges, so when you see them at ah you simply assume that they have all 5 charges.
However upon checking the mailbox I noticed that it only had one charge left. So what did I do? I put it up on the ah again for 25g. It was sold after 3 hours. When using the auctioneer list, there is no way to see how many charges a mana oil has left, and even using normal search the chance is high that the buyer will assume that it's a full oil with 5 charges.
Now it would be complicated to actually make any money out of this, but as a high end raider I can spend over 100g a week on just mana oils, and getting most of that refunded is sort of nice.