Pussycat Catnap has a post that expresses some of my misgivings over Marketplace very well.
Why moving a shop to Marketplace ‘hurts’ the SL economy..
I try my best to shop in-world whenever possible. Often I will do without rather than buy sight unseen on Marketplace.
The only use I really have for Marketplace is as a product index, not a perfect one by any means, but way better than in world search.