I was at the bazzar once and I clicked on something and then I clicked on BUY and a note appeared that said your transaction was not successful and i clicked on another thing and went back to what i wanted to buy and then it was not there! It happened a lot and prevented me from getting a helephant treasure card!
A quick search of these message boards would have given you your answer, young Wizard.
You are not the only one shopping at the Bazaar - all the items in the Bazaar have been sold by other players, and other players in all the realms are shopping the same Bazaar as you.
When you tried to buy the item and were not successful, it means that someone else bought that item before you did, which is why when you went back to the inventory window - the item was gone.
I was at the bazzar once and I clicked on something and then I clicked on BUY and a note appeared that said your transaction was not successful and i clicked on another thing and went back to what i wanted to buy and then it was not there! It happened a lot and prevented me from getting a helephant treasure card!
Did it have a number ONE by the side of it? If so, someone all ready bought it. No reason to woe.
In addition to the Professor's reply, I would suggest one day when you have time, just hang out at the bazaar and observe the products for a bit. The way things move through there would amaze you.