I've never bought clothes (dresses, blouses, jeans, shoes) online, and I doubt I ever will. I'm very particular about the way things fit, and I would hate to go through the hassle of returning things when/if the y don't fit.
I have, however, bought t-shirts and hooded sweatshirts online, and have never had a problem. Those don't typically have a wide range of different "fits" though lol.
I've started buying stuff online, but only from stores that I've tried stuff on in person, because some things just fit...weirdly...that I'd swear should be fine. Banana Republic pants come to mind. They just look really bad on me, but I love their shirts. Express has the perfect pants for me, but their shirts don't seem to be made for girls with any type of chest.