If the store provides you a valuable service, like allowing you to try on shoes or giving you significant advice on which product to buy, you should buy the product from them so that they get profits with which to cover the cost of providing that service.
Now, it depends how extensively you used their resources and how much of a markup they want. If you were in the store for something else and all you did is wander around to try to get ideas of what else you wanted, I don't think there's much obligation to buy. But if you're spending 15 minutes talking to a salesman about all of the possibilities (especially if you wanted to ask him the questions, as opposed to him seeing you in the area and approaching you to try to close the sale and upsell you) or extensively trying out the demo units, the store is providing you value and you're using resources that cost them money. As long as the additional markup that the store wants is fair (for an example of unfair markup see the guy who chose not to buy a $40 DVI-HDMI cable that costs <$3 on monoprice), you should compensate them for the value they provided you and buy from them.