This question is acutely relevant today. You are shopping for something. Your plan is to go to a lot of brick-and-mortar stores, see the item, feel it, ask the salespersons lots of questions, then go home and order it on the Internet at a discount and save the tax. I know the halacha is that "you don't ask for the price of an item if you have no intention of buying". But here you are not just asking for the price. You might buy it in the store. The store should be aware of the competition (including on the Internet), can offer servicing in the price, etc. You just don't know. But the fact is that, most of the time, you end up buying it on the Internet. Is this ethical? If not, how is that different from plain shopping, with the Internet being just one of the venues for sellers?
From an article by Rabbi Meir Orlian, answering this question
It is permissible to browse, though, if there is a possibility that you might buy there. Any store owner knows that potential customers comparison shop and might decide not to buy there. (Pischei Choshen, Ona’ah 15 nt. 15) It is also permissible if you ask the store owner up front, “Do you mind if I browse the products without buying?”
Furthermore, browsing in a large store full of customers is permissible if you do not distract the salespeople, since the owner or salespeople do not note an individual person who enters and browses. Similarly, it is permissible to browse in stores that emphasize feature displays, such as FAO Schwarz, since the owner encourages people to view the display and does not necessarily expect a sale.
Finally, the prohibition of ona’as devarim applies only to a fellow Jewish storeowner. (Rama 228:1)