From an international (other side of the world) point of view it absolutely comes down to two things - overall cost of an item including shipping, customs, duties, tax etc - often it is cheaper to pay all of that than buy domestically (but often I will buy domestically if it is a good independent store).
Many items that I try and source are just not available here. For example, based on a couple of demos overseas in January and knowledge of what is coming out for this season I have narrowed my demo list for new skis down, with the Nordica Santa Ana, blizzard Samba and I want to try a couple of the unisex blizzards including the new bonafide. But, it turns out that it is the same importer for both blizzard and nordica - and they are not bringing in either the Santa Ana or the Samba this year - so online becomes an option, or waiting until we travel overseas again. Sometimes, try as you might you just can't buy what you want locally in a small market