Nice ideas, although i wouldnt vote for buying immediately from the search menu; that would eliminate the interaction with retainers and the physical trade feel of the ward.

I like how everyone searching for an item have the chance to run to the retainers and buy the goods, instead of everyone standing upon an AH. Maybe that's a personal taste.

In the first suggestion, i could add that upon having a list of all items, players get the ability to click on one (say the cheapest) which basically marks that retainer with special star (say yellow colored) instead of red that represents everyone who sells the same item. That's a small addition, but i believe it helps leveraging your main idea.