At the risk of sounding vapid, one reason I keep returning to Barcelona is that nice clothes are sold there.
Plus, I get to buy caganers in Barcelona. But I could get those mail-order.
But I can't buy clothes from the small Barcelona designers online. Maybe someday, but not yet.
My absolute favorite Barcelona designer is named Boyd Baten and his clothes are sold at a little shop called Bionic on Petritxol (#18), a tiny alley between Plaza del Pi and Portaferrissa (which is home to a number of low-cost chain clothing stores such as H&M).
I've never found anything at the Mango Outlet, or at Zara or Camper, but I do sometimes stumble over something nice at Il Corte Ingles, the mega-department store on Plaza Catalunya.
I buy cool European Pumas on Jaume, to the east of Avinyo, but damned if I know the name of the store. It's a generic sneaker shop. All I know if that it sells Pumas we don't get in the States, so I always stop there. Not that I'm some huge Puma collector—but I like to have some super-lightweight, comfy shoes in my pack. From there, I always stroll down Avinyo, stopping in all the boutiques along the way.
My other don't-miss clothing stores:
Fete makes interesting—cheap—T-shirts and long-sleeve shirts. I go to their outlet shop at Princesa 13 near Argenteria.
On Land is about a half-block further down Princesa, at #25. It is NOT cheap. But it has a lot of lovely clothing. Sometimes I even buy some instead of just browsing.
Anna Povo in Born always gets a visit from me, in spite of having more girly clothes than I usually wear. But they are lovely, not too expensive, and best of all, the shop is open on Sundays.