I'm going to go against the grain here, and say that I would not like this as a feature.
It's already a pain to keep my merchant stocked, and it's already doubly not pleasant to learn that someone has purchased the entirety of something I just stocked.
I think this would encourage inflation way too much in our already tiny community. People who want to do quests, or even just have items will have a harder time getting things when you can buy a shop's entire stock in a second flat, just because you can.
If its introduced, it should be something that's off at launch, and has to be turned on by the shop owner.