So, LL has acquired XStreetSL and OnRez. In a curious parallel to RL, there is much talk about how this will affect prim-and-mortar stores.
Initially, not much, I'd say. Only a fifth of "heavy Second Life users" regularly shop via XStreetSL. As others have pointed out, shopping in SL can be a social activity; also, there's nothing like actually seeing something you're thinking of buying. Photos of clothing can hide a multitude of sins that the careful shopper can check for on an in-store model. ("I wonder what this looks like at midnight/with Torley's Watermelon Sky Windlight setting/etc....") Were I in the market for a house or furniture, I'd certainly want to walk through it or sit on it.
On the other hand, it can be amazingly difficult to find something even if you know the seller, or even if you've seen it before and are trying to find it again! If they can provide a decent search, it will be great for comparison shopping--though in a reversal of RL, that might mean people will use the web site to narrow the possibilities down, then go to the prim-and-mortar store for a final inspection and purchase. (That doesn't bode well for SL malls.) Also, people might go to a web site purely out of frustration at waiting for textures to trickle into the battleship gray world that SL stores all too often are when one teleports to them.
It will be interesting to see whether SL residents shop more via the web site that results--if they don't, can LL afford to keep it going? I would seriously consider subscribing to a web site that provides a really good search and comparison shopping facility, but would still go to stores in SL for the final purchase.