I personally like to search out the item i'm after, and then go shopping around. Hell, the npc i get the item from itself will have plenty of other stuff i wasn't searching for too, sometimes i buy more than i'd intended.

The idea behind this is infact that people will do this, impulse purchases and such, and once done, they may even browse other retainers, with an AH, you just don't tend to impulse buy, not unless you see something you wanted anyway near what you were searching for.